Katie Jill O'Brien (born 2 May 1986) is a British former professional tennis player from Beverley, Yorkshire. She was briefly the British No. 1 tennis player, and reached her career-high singles ranking of world No. 84 on 1 February 2010. She won four singles and two doubles titles on the ITF Women's Circuit.

In 2007, she reached the second round of her home Grand Slam, Wimbledon, by beating Sandra Klösel in round one. She lost to the No. 31 seed, Michaëlla Krajicek, in the second round. In 2010, she replicated this achievement by beating Patricia Mayr to reach the second round of the Australian Open where she fell to eighth seeded Jelena Janković.The Press Association, 19 January 2010

Personal life O'Brien's parents are Phil and Jill O'Brien. Her father is a quantity surveyor, and her mother is a nursery school teacher. Her brother James, and sister, Holly, both have been awarded the Development Coach Award, a licence to coach tennis from the Lawn Tennis Association.

O'Brien attended Hymers College, Hull from 1997 to 2002. She studied her A-levels at Woodhouse Grove School in Leeds from 2002 to 2004. She began studying business studies with the Open University in 2008, pursuing studies while competing professionally.

Career Juniors (2000–2004) O'Brien played her first match on the ITF Junior Circuit in February 2000 and her last in June 2004. During her junior career, she won two titles—the Team Leschly Danish Junior Cup in February 2002 and the Vierumaki Junior Cup in October the same year. She also reached a total of five semifinals and three quarterfinals overall. Her best result in a junior major tournament came in the 2003 Wimbledon Championships where she reached round three, before losing to Emma Laine, 1–6, 5–7. Her career-high junior singles ranking was world No. 101, reached on 15 September 2003, and her overall win–loss record in singles was 44–34.

In addition to her singles success as a junior, she also won one doubles title, lost in the final of another, reached four doubles semifinals and seven doubles quarterfinals (one of which in the 2004 Wimbledon Championships, partnering compatriot Melanie South). She reached her career–high doubles ranking of world No. 201 on 3 February 2003, and ended her junior career with a win–loss record of 17–26.

2001–2004 O'Brien played her first match on the ITF Women's Circuit in September 2001 when she attempted to qualify for a $10k event in Sunderland. She lost in the second round of qualifying for this tournament.

One year later, in September 2002, she reached her first quarterfinal as a qualifier in the $10k event in Sunderland and was beaten by fellow Brit Anne Keothavong, 6–0, 6–1. This was her only quarterfinal appearance in 2002 and she finished the year with a ranking of world No. 693.

In 2003 she again reached only one quarterfinal, this one in the $10k tournament in Felixstowe. In every other tournament she played she either lost in the first round or the qualifying stages. As a result, her year-end ranking fell from the previous year to world No. 742.

In May 2004, O'Brien played her first ever semifinal on the ITF Circuit against Elke Clijsters (the sister of former world No. 1 Kim Clijsters) and was beaten 6–3, 7–5. She was given a wildcard into the main draw of her home Grand Slam tournament in Wimbledon for the first time in her career in June 2004. She lost in the first round, 4–6, 4–6 to María Sánchez Lorenzo. Following Wimbledon, she played twelve more tournaments on the ITF Circuit, reaching the final of one (Manchester, $10k), the semifinals of three of them and the quarterfinals of two more. Her end-of-year ranking was world No. 401.

2005 The 2005 season started well for O'Brien; she reached the finals of her first two ITF tournaments (in Tipton and Hull) and won the second of the two. In her third tournament, Sunderland, she reached the quarterfinals but was beaten by compatriot, Elena Baltacha, 6–4, 6–3. Her second quarterfinal of the year was in Oxford.

In June, she received three wildcards into three consecutive WTA Tour tournaments; the Birmingham Classic qualifying draw, the Eastbourne International Championships qualifying draw and Wimbledon main draw. She failed to qualify in the first two tournaments and lost to Kim Clijsters, 2–6, 3–6, in the first round of Wimbledon.

Following Wimbledon she entered the $25k tournament in Felixstowe and reached the semifinals before losing to Jarmila Gajdošová, 4–6, 4–6. She competed in twelve more ITF tournaments that year (reaching the quarterfinals in six of them) and the qualifying tournament for the Luxembourg Open where she reached the second round of qualifying. She finished the 2005 season with a ranking of world No. 263.

2006 In January, O'Brien reached yet another ITF quarterfinal, this one in Hull, and the very next month she progressed to the semifinals of the $25k tournament in Sunderland, before easily losing to Anne Keothavong in two sets. She reached the semifinals of the $50k event in Fukuoka, Japan where she was beaten by Ayumi Morita.

She again spent June attempting to qualify for the Birmingham Classic and the Eastbourne International without success. She also competed in the Wimbledon main draw but again lost in round one, against Tathiana Garbin in three sets.

After this she lost to Monique Adamczak in the quarterfinals of the $25k event in Valladolid in July and she reached the final of the ITF tournament in Changsha ($25k) in August. Two consecutive $25k finals followed in September; the second of which she won. Her good form continued into October and November when she reached the quarterfinals, semifinals and quarterfinals of $25k tournaments in Glasgow, Istanbul and Jakarta, respectively. Her final ranking of 2006 was world No. 193.

2007 O'Brien's first tournament of the 2007 season was the qualifying event for the Australian Open where she beat Chen Yanchong in a marathon three-setter (3–6, 6–2, 8–7 ret.) to set up a second-round clash with Anastasia Pavlyuchenkova which she lost. She then returned to the ITF Circuit until May. During this time she played in ten ITF tournaments, reaching the quarterfinal stages in four of them (two $50k events and two $25k events). She then attempted to qualify for the French Open but was beaten by Florence Haring, in three sets.

O'Brien's grass court season began with another ITF quarterfinal, this one in Surbiton ($25k). She then tried to qualify for the Tier III tournament, the Birmingham Classic but was beaten in another tough three-set match, 7–6, 5–7, 7–6, by compatriot Sarah Borwell. O'Brien had more success trying to qualify for the Tier II Eastbourne International where she beat Yuan Meng, Tsvetana Pironkova and Jill Craybas to qualify. She faced Vasilisa Bardina in the first round of the main tournament and had enough momentum carried over from the qualifying rounds to defeat her as well, 7–5, 6–3. Her opponent in the round of 16 was No. 4 seed, Elena Dementieva who proved too much for the Brit. She beat O'Brien by a score of 6–3, 6–4. The confidence and experience she gained helped her capitalise on a wildcard into the main draw of Wimbledon; she beat Sandra Klösel in round one, 6–3, 7–5, (by coming back from 3–5 down in the second set) before being demolished in the second round by 31st seed, Michaëlla Krajicek, 6–0, 6–1.

O'Brien spent August trying to qualify for the Carson Classic (Tier II), the Roger's Cup (Tier I) and the final major of the year, the US Open. She failed to win a match in any of these qualifying tournaments. In September she managed to reach the final round of qualifying in a Tier IV WTA tournament, the Slovenia Open in Portorož and just a few weeks later reached the quarterfinals of the main draw of a Tier IV event, the Tashkent Open, for the first time in her career before losing to Olga Govortsova, 2–6, 3–6. She played six more $25k tournaments that year, reaching the semifinals in four of them and the final of another. She finished 2007 with a ranking of world No. 134.

2008 O'Brien's first tournament of the season was the Australian Open where she entered qualifying as the No. 15 seed. After being gifted an easy first round victory over Darya Kustova when Kustova had to retire with the score at 6–3, 1–0 in O'Brien's favour, O'Brien was forced to fight hard in her second qualifying match against Yanina Wickmayer. She eventually won, 4–6, 6–3, 8–6, to set up a final round qualifying match against the No. 11 seed in the qualifying tournament, Yuan Meng. O'Brien was defeated 6–4, 6–2. In early February, she competed as part of the Great Britain Fed Cup team in the Fed Cup and was given tough opponents. She challenged Patty Schnyder but eventually lost, 6–7, 5–7, and also lost to Ágnes Szávay and Caroline Wozniacki. However, she and Anne Keothavong managed to save Britain from relegation from the Europe/Africa Group I by winning two singles matches against Portugal. This was followed by O'Brien's first appearance on the ITF Circuit in 2008 which came at the $75k event in Midland where she was the No. 5 seed. She faced the No. 2 seed, Laura Granville, in the quarterfinals and was beaten, 1–6, 6–3, 6–1. O'Brien then attempted to qualify for four consecutive WTA Tour events, in Doha, Dubai, Bangalore and Indian Wells. She lost in round one of qualifying in each of them with the exception of Indian Wells where she lost in the round two of qualifying to Barbora Záhlavová-Strýcová. Over March and April she reached the quarterfinals of two ITF tournaments: Jersey ($25k) and Saint-Malo ($100k). In May, O'Brien entered the French Open qualifying draw and beat fellow Brit, Melanie South in the opening round. She fell in round two to Kristina Barrois, 4–6, 2–6, and then headed to Birmingham where she beat Alla Kudryavtseva, before falling to Yaroslava Shvedova in round two. A wildcard allowed O'Brien entry into the International Women's Open (previously the Hastings Direct International) where she lost to Samantha Stosur. Shahar Pe'er beat O'Brien then in the first round of Wimbledon.

Following the third major of the season, O'Brien participated in the Internazionali di Palermo where Lourdes Domínguez Lino beat O'Brien in round one. Two weeks later she reached the second round of the Slovenia Open by beating No. 5 seed, Tsvetana Pironkova, in round one. She lost to Vera Dushevina in the second round. In August, she reached the second round of qualifying for the US Open before being knocked out by home favourite, Alexa Glatch. September saw O'Brien fall to Raluca Olaru in round one of the Tashkent Open and in October she reached the semifinals of the $50k event in Barnstaple where she lost to Anne Keothavong. She reached two more ITF quarterfinals in 2008, in Phoenix and Saint-Denis, both $25k. Her year-end ranking was world No. 154.

2009