FRANKFURT, NOV. 18 -- Stefan Edberg was No. 1 for 1990. Sunday, he had to settle for being second-best.

Andre Agassi, playing perhaps the best tennis of his career, defeated Edberg, the defending champion, in four sets to win the ATP World Championships.

"It's unbelievable to accomplish this at my age," said Agassi, 20. "It's very special. . . . There is no question that this is the high point of my career. Stefan has been playing the best tennis all year and the pressure wasn't on me."

In avenging an earlier defeat to Edberg -- who was assured of the year's top ranking by making the final -- during the round-robin the tournament, Agassi earned $600,000 for his 5-7, 7-6, 7-5, 6-2 victory. In all, Agassi collected $950,000 from the $2 million purse in the eight-player tournament.

After winning the second-set tiebreaker, Agassi dominated the match and broke three times in the last set to win in 3 hours 15 minutes. The American tossed his racket into the stands after Edberg hit a backhand into the net on match point.

Agassi said he had been battling a cold for two days.

"At 4-2 in the first set I thought there was no way I could win, I felt so horrible, so drained," he said.

Agassi became the first American to capture the season-ending championship since John McEnroe in 1985, when the event was called the Masters. He also became the second-youngest winner; McEnroe won the first of his three Masters titles in 1979 at 19.

"When you win an event with top players in the world, that gives you confidence," said Agassi, who lost in the finals at this year's U.S. and French Opens. "This win should make some people believers."

It was the fourth title for Agassi in seven finals this year and his first victory over Edberg on carpet. The Swede leads the series 3-2, but was beaten once by Agassi earlier this year, at Key Biscayne, Fla.

Edberg, who won seven tournaments this year, had beaten Agassi in three close sets during this event.

"I could not come up with the right shots when I needed them," Edberg said. "He was very consistent. He returned very well throughout and hit the ball very hard."

"I was a bit tight and he's had a very good week," Edberg said.

Agassi beat Boris Becker, ranked No. 2 in the world, on Saturday to reach the final, while Edberg eliminated Ivan Lendl, the No. 3 seed.