Katie Jill O'Brien (born 2 May 1986) is a British former professional tennis player from Beverley, Yorkshire.

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.

Her best result in a junior Grand Slam tournament came in the 2003 Wimbledon Championships where she reached round three before losing to Emma Laine, 1–6, 5–7.

[6] 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.

[7] One year later, in September 2002, she reached her first ITF quarterfinal as a qualifier in the $10k event in Sunderland and was beaten by fellow Brit Anne Keothavong, 6–0, 6–1.

She was given a wildcard into the main draw of her home Grand Slam, Wimbledon for the first time in her career in June 2004.

[8] 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.

[7] 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.

She reached the semifinals of the ITF event in Fukuoka, Japan ($50k) where she was beaten by Ayumi Morita.

She again spent June attempting to qualify for the DFS Classic and the Hastings Direct International Championships without success.

She also competed in the Wimbledon main draw but again lost in round one, beaten by Tathiana Garbin in three sets, 2–6, 7–6(2), 6–2.

[10] 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.

[7] 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.)

She then tried to qualify for the Tier III tournament, the DFS Classic but was beaten in another tough three set match, 7–6, 5–7, 7–6, by compatriot Sarah Borwell.

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.

However, she and Anne Keothavong managed to save Britain from relegation from the Europe/Africa Group I by winning two singles matches against Portugal.

O'Brien then attempted to qualify for four consecutive WTA events, in Doha, Dubai, Bangalore and Indian Wells.

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 Ioana Raluca Olaru in round one of the Tashkent Open and in October she reached the semifinals of the $50k in Barnstaple where she lost to Anne Keothavong.

[7] O'Brien started her 2009 season by attempting to qualify for the ASB Classic in Auckland, New Zealand.

She beat Sandra Záhlavová, Julie Ditty and Betina Jozami to gain entry into the main draw for the first time in her career.

O'Brien's three compatriots were Anne Keothavong, Melanie South and fellow qualifier, Elena Baltacha.

In May she reached the final round of qualifying for the French Open before losing to Arantxa Rus, 4–6, 6–7, however she gained entry to the main draw as a lucky loser when No.

She was then given a wildcard into the main draw of the Aegon Classic where she was drawn to face fellow British wild card, Melanie South.

Another wildcard then granted O'Brien entry to the main draw of Wimbledon where she lost, 2–6, 7–5, 4–6, in round one to world No.

[21] Over the course of her final few tournaments of the year, O'Brien reached another $100k semifinal as well as the second round of the HP Open where she was swept aside 6–0, 6–1 by top seed Caroline Wozniacki.

[7] O'Brien started 2010 by attempting to qualify for the WTA ASB Classic in Auckland but lost her opening match 1–6, 5–7 to Canadian Valérie Tétreault.

She moved onto the WTA Premier event in Sydney and lost 3–6, 3–6 to Anastasia Rodionova in the first round of qualifying.

[22] In the second round, O'Brien faced the eighth seed and former world number one, Jelena Janković, and put in a spirited performance but went down 2–6, 2–6.