Question: What is the name of this retired professional tennis player and former World No. 1 who was the defending champion of the 2004 Siebel Open - Singles?

Context paragraphs:
2004 Siebel Open – Singles: Andre Agassi was the defending champion but lost in the semifinals to Mardy Fish.

2004 Siebel Open: The 2004 Siebel Open was a tennis tournament played on indoor hard courts at the HP Pavilion at San Jose in San Jose, California in the United States that was part of the International Series of the 2004 ATP Tour.  The tournament ran from February 9 through February 15, 2004.

Juan Carlos Ferrero: Juan Carlos Ferrero Donat (] ; born 12 February 1980) is a retired professional tennis player, and a former world No. 1 player, from Spain.  He captured the men's singles title at the 2003 French Open, and in September of that year, became the 21st player to hold the world No. 1 ranking.  He was also the runner-up at the 2002 French Open and the 2003 US Open.  His nickname is "Mosquito" due to his speed and his slight physical build.  Ferrero retired from the game after the 2012 Valencia Open 500.

Juan Martín del Potro: Juan Martín del Potro (] , born 23 September 1988), also known as Delpo is an Argentinian professional tennis player who is currently ranked world No. 24 in men's singles by the Association of Tennis Professionals (ATP).  His biggest achievement has been winning the 2009 US Open, defeating Rafael Nadal in the semifinal and 5-time defending champion Roger Federer in the final.  He was the first to defeat both Federer and Nadal during the same major and was the only man outside the Big Four to win a major between the 2005 French Open and the 2013 US Open, a span of 35 tournaments.  He is also the second Argentine and the fifth-youngest man to win the US Open in the Open Era.  Other career highlights include winning the bronze medal in men's singles at the 2012 London Olympics and the silver medal at the 2016 Rio Olympics and being part of his country's successful Davis Cup team; but his career has also been hampered by a succession of wrist injuries.

Lucie Šafářová: Lucie Šafářová (] ; born 4 February 1987) is a Czech professional tennis player from Brno.  She is currently the WTA world No. 1 doubles player.  Šafářová has won seven WTA tour singles titles, fifteen doubles titles and reached the quarterfinals of the 2007 Australian Open by upsetting defending champion Amélie Mauresmo in the fourth round.  She reached her first Grand Slam semifinal at the 2014 Wimbledon Championships and her first Grand Slam final at the 2015 French Open.  She also won the 2015 Australian Open, 2015 French Open, 2016 US Open, 2017 Australian Open and 2017 French Open women's doubles titles partnering with Bethanie Mattek-Sands.

Tracy Austin: Tracy Ann Austin Holt (born December 12, 1962) is an American former World No. 1 retired professional tennis player.  She won three Grand Slam titles, including the women's singles titles at the 1979 and 1981 US Opens, and the mixed doubles title at the Wimbledon Championships in 1980.  Additionally, she won the WTA Tour Championships in 1980 and the year-ending Toyota Championships in 1981, both in singles.  A series of injuries and a serious automobile accident cut short her career.  Since 1979, she has been the youngest US Open female singles champion in history, and she is the youngest inductee of all time at the International Tennis Hall of Fame.  Austin won singles titles on all playing surfaces: clay (both red clay and green clay), indoor carpet, grass, and hard courts.

2002 Siebel Open – Singles: The 2002 Siebel Open was an international tennis tournament that took place from February 25 to March 3, 2002.  Greg Rusedski was the defending champion but lost in the second round to Todd Martin.

Li Na: Li Na (; ; born 26 February 1982) is a retired Chinese professional tennis player, who achieved a career-high WTA-ranking of world No. 2 on 17 February 2014.  Over the course of her career, Li won seven WTA singles titles and two Grand Slam singles titles at the 2011 French Open and 2014 Australian Open.  Li's rise to prominence came after those victories, which made her the first and only Grand Slam singles champion from East Asia and Asia as a whole.  Prior to this, she had already become the first player representing an East Asian and Asian country to appear in a Grand Slam singles final, a milestone she achieved at the 2011 Australian Open.  Li was also the runner-up at the 2013 Australian Open and 2013 WTA Tour Championships, a three-time quarterfinalist at Wimbledon and a semifinalist at the 2008 Beijing Olympic Games and 2013 US Open.  Among her other most notable accolades, she was the first Chinese player to win a WTA tour title at the Guangzhou International Women's Open in 2004, the first to reach a Grand Slam singles quarterfinal at the 2006 Wimbledon Championships, and the first to break into the world's top 10.  Her feats have sparked a major population growth of tennis players in East Asia, earning her the reputation as the region's tennis pioneer and trailblazer.

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.

Marta Domachowska: Marta Domachowska (; born 16 January 1986 in Warsaw) is a Polish retired professional tennis player ranked World No. 37 in singles (2006) and World No. 62 in doubles (2006).  She reached 2008 Australian Open fourth round in singles and won 2006 Canberra International in doubles with Roberta Vinci.  She also reached three WTA Tour singles finals at 2004 Hansol Korea Open (lost to Maria Sharapova), 2005 Internationaux de Strasbourg (lost to Anabel Medina Garrigues) and 2006 U.S. National Indoor Tennis Championships (lost to Sofia Arvidsson).  She was 2003 Australian Open finalist in girls' singles, represented Poland at the 2008 Summer Olympics and was member of Poland Fed Cup team.  Domachowska was the best female Polish tennis player after Magdalena Grzybowska's retirement and before Agnieszka Radwańska's successes.

Supporting paragraphs: 2004 Siebel Open – Singles Andre Agassi
Answer: Andre Agassi