In 1998, Agassi began a rigorous conditioning program and worked his way back up the rankings by playing in Challenger Series tournaments, a circuit for pro players ranked outside the world's top 50. He also played classic matches in this period against Pete Sampras and Patrick Rafter.
In 1998, Agassi won five titles and leapt from World No. 122 to No. 6, the highest jump into the top 10 made by any player during a calendar year.[27] At Wimbledon, he had an early loss in the second round to ATP player Tommy Haas. He won five titles in ten finals and was runner-up at the Masters Series tournament in Key Biscayne, losing to Marcelo Ríos, who became World No. 1 as a result.
Agassi entered the history books in 1999 when he came back from two sets to love down to beat Andrei Medvedev in a five-set French Open final, becoming, at the time, only the fifth male player (joining Rod Laver, Fred Perry, Roy Emerson and Don Budge—these have since been joined by a sixth, Roger Federer and a seventh, Rafael Nadal) to have won all four Grand Slam singles titles during his career. This win also made him the first (of only three, the second being Roger Federer and the third being Rafael Nadal) male players in history to have won all four Grand Slam titles on three different surfaces (clay, grass, and hard courts), a tribute to his adaptability, as the other four men had won their Grand Slam titles on clay and grass courts. Agassi also became the first (of only two, the second being Rafael Nadal) male player to win the Career Golden Slam, consisting of all four Grand Slam tournaments plus an Olympic gold medal.
Agassi followed his 1999 French Open victory by reaching the Wimbledon final, where he lost to Sampras in straight sets.[21] He rebounded from his Wimbledon defeat by winning the US Open, beating Todd Martin in five sets (rallying from a 2 sets to 1 deficit) in the final. Agassi ended 1999 as the World No. 1, ending Sampras's record of six consecutive year-ending top rankings (1993–1998).[21] This was the only time Agassi ended the year at number one.
Agassi began the next year by capturing his second Australian Open title, beating Sampras in a five-set semifinal and Yevgeny Kafelnikov in a four-set final.[21] He was the first male player to have reached four consecutive Grand Slam finals since Rod Laver achieved the Grand Slam in 1969.[28] At the time, Agassi was also only the fourth player since Laver to be the reigning champion of three of four Grand Slam events, missing only the Wimbledon title.[29]
2000 also saw Agassi reach the semifinals at Wimbledon, where he lost in five sets to Rafter in a match considered by many to be one of the best ever at Wimbledon.[30] At the inaugural Tennis Masters Cup in Lisbon, Agassi reached the final after defeating Marat Safin 6–3, 6–3 in the semifinals to end the Russian's hopes to become the youngest World No. 1 in the history of tennis. Agassi then lost to Gustavo Kuerten in the final, allowing Kuerten to be crowned year-end World No. 1.
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