Question: Was Kristie Boogert or Alex O'Brien ranked higher as a singles player?

full_context: Document 1: Rick Leach: Rick Leach (born December 28, 1964) is a former professional tennis player and a coach from the United States. A doubles specialist, he won five Grand Slam men's doubles titles (three Australian Open, one Wimbledon, one US Open), and four Grand Slam mixed doubles titles (two Australian Open, one Wimbledon, one US Open). Leach reached the World No. 1 doubles ranking in 1990.
Document 2: 2001 Indian Wells Masters – Men's Doubles: Alex O'Brien and Jared Palmer were the defending champions but they competed with different partners that year, O'Brien with Sébastien Lareau and Palmer with Byron Black.
Document 3: Bob Bryan: Robert Charles Bryan (born April 29, 1978) is an American male professional tennis player. He has won twenty-three Grand Slam titles: 16 in men's doubles and 7 in mixed doubles. He turned professional in 1998. With his twin brother Mike, he has been world No. 1 doubles player for much of the last several years, first achieving the top ranking in September 2003. The brothers were named ATP Team of the Decade for 2000–2009. The brothers became the second men's doubles team to complete the career golden slam at the 2012 Summer Olympics.
Document 4: Kristie Boogert: She has not won any singles titles, but did win three titles in doubles. At the 1994 French Open championships she won the mixed doubles title with countryman Menno Oosting.
Document 5: Jim Courier: James Spencer Courier Jr. (born August 17, 1970) is an American former World No. 1 professional tennis player. During his career, he won four Grand Slam singles titles, two at the French Open and two at the Australian Open. He holds the record for being the youngest man to have reached the finals of all four Grand Slam singles tournaments, at the age of 22 years and 11 months. He also won five Masters 1000 series titles. Until Novak Djokovic in 2016, Courier was the last man to win both the Australian and French Opens in the same calendar year.
Document 6: 1997 Open Gaz de France – Doubles: Kristie Boogert and Jana Novotná were the defending champions but they competed with different partners that year, Boogert with Irina Spîrlea and Novotná with Martina Hingis.
Document 7: 1997 SEAT Open – Doubles: Kristie Boogert and Nathalie Tauziat were the defending champions but only Boogert competed that year with Amanda Coetzer.
Document 8: 2011 Büschl Open – Doubles: Kristina Barrois and Anna-Lena Grönefeld were the defending champions, but Grönefeld decided not to participate. Barrois partnered up with Yvonne Meusburger, but lost in the final to Kiki Bertens and Anne Keothavong.
Document 9: Jamie Murray career statistics: Jamie Murray is a professional tennis player who is the current British number one doubles player. He has reached eight grand slam finals in total: (4 Doubles, 4 Mixed), he has won the mixed doubles at the 2007 Wimbledon Championships, 2017 Wimbledon Championships and 2017 US Open, and the men's doubles at the 2016 Australian Open and 2016 US Open, and has finished as runner-up in the men's doubles tournament at the 2015 Wimbledon Championships and 2015 US Open and in mixed doubles at the 2008 US Open. Murray has been ranked as high as World No. 1 in the ATP doubles rankings, and was the first Britain to be ranked as world number one since the introduction of computerised world rankings in the 1970s. He is currently ranked at world No. 5.
Document 10: 1997 Sparkassen Cup – Doubles: Kristie Boogert and Nathalie Tauziat were the defending champions but only Boogert competed that year with Amanda Coetzer.
Document 11: Bent-Ove Pedersen: Bent-Ove Pedersen (born 11 July 1967 in Oslo) is a former tennis player from Norway, who turned professional in 1992. He spent several years playing tennis at Berkeley in California. The right-hander represented his native country in the doubles competition at the 1992 Summer Olympics in Barcelona, where he partnered Christian Ruud. The pair was defeated in the first round by South Africa's eventual runners-up Wayne Ferreira and Piet Norval. Pedersen reached his highest singles ATP-ranking on 13 September 1993, when he became the number 366 of the world. He was in quarterfinal in US Open doubles, 1991, partnering Matt Lucena from the US. He became the number 78 on the doubles ranking 30. August 1993.
Document 12: 2000 Australian Open – Mixed Doubles: Mariaan de Swardt and David Adams were the defending champions but only Adams competed that year with Kristie Boogert.
Document 13: Alex O'Brien: Alex O'Brien (born March 7, 1970 in Amarillo, Texas) is a retired professional tennis player from the United States, who competed on the ATP Tour. He became the world No. 1 doubles player in May 2000 and was ranked as high as world No. 30 in singles in June 1997.
Document 14: 1996 SEAT Open – Doubles: Kristie Boogert and Nathalie Tauziat won in the final 2–6, 6–4, 6–2 against Barbara Rittner and Dominique Van Roost.
Document 15: Andre Agassi: Andre Kirk Agassi ( ; born April 29, 1970) is an American retired professional tennis player and former World No. 1 who was one of the sport's most dominant players from the early 1990s to the mid-2000s. Generally considered by critics and fellow players to be one of the greatest tennis players of all time, Agassi has been called the greatest service returner in the history of the game. Described by the BBC upon his retirement as "perhaps the biggest worldwide star in the sport's history", Agassi compiled performances that, along with his unorthodox apparel and attitude, saw him cited as one of the most charismatic players in history. As a result, he is credited for helping to revive the popularity of tennis during the 1990s.
ground_truth: No. 30 in singles