Boris Becker won his first tournament since August and ended Ivan Lendl's 29-match winning streak with a 7-6 (7-5), 6-3 victory yesterday in the championship match of the Chicago Grand Prix tournament.

Becker, the 1985 Wimbledon champion who had lost to Lendl four previous times, won $50,000. Lendl, the No. 1 player in the world, had won nine of his last 10 tournaments and four in a row and had not lost indoors in more than a year. He earned $25,000.

Lendl, who has suffered from tendinitis in his knee for the last month, aggravated the ailment midway through the first set and had difficulty running. He said he will not play again until the problem is taken care of.

"It was giving me trouble on low balls to my backhand," Lendl said. "The same thing for my forehand when I run. I have to use my upper body and not my lower body."

Becker, 18, said he did not notice Lendl having trouble. "I don't think it bothered him that much," Becker said. "He aced me at 6-4 in the tie breaker."

After beating Lendl in the first-set tie breaker, Becker broke serve in the second set to lead, 4-3. He held serve, then broke Lendl in the ninth game to finish the match.

The end was surprisingly sudden. Lendl missed a drop shot from the base line, then double-faulted. Becker hit a service return winner to go ahead, 0-40. Lendl won two points before hitting a forehand wide.

In the first set, Becker got the first break point in the sixth game by diving at the net to retrieve a forehand. But Lendl's serve-and-volley winner forced deuce and he won the game to tie the set at 3.

Becker escaped a break point when Lendl made a forehand error to hold serve, then broke Lendl to take a 5-3 lead.

In losing his serve in the eighth game, Lendl missed two forehands to go down by 15-30, then Becker got lucky when a forehand tipped the net and fell over. He won the game with a backhand passing shot, but turned around and lost his next serve by netting a service volley. They stayed on serve to force the tie breaker.

In the tie breaker, Lendl netted a backhand and went wide with a backhand return in falling behind, 6-3. He closed the score to 6-5 with a service winner and the match's first ace before hitting a lob long to lose the set.

Becker, No. 5 in the world, lost in the first round last week, which is the reason he decided to play in Chicago. "Sometimes I lose early, then next week I beat everybody," he said. "Consistency comes with age. But I belong in the top 10 for sure." . . .

The No. 8 player in the world, Joakim Nystrom, won the ABN tournament, defeating Anders Jarryd, 6-0, 6-3, in Rotterdam.

Third-seeded Nystrom, 23, had an easy time in the 90-minute match until his first match point at 5-1 in the second set.

Fourth-seeded Jarryd, who had lost the first nine games of the match, rallied to cut his second-set deficit to 5-3. But, Nystrom broke Jarryd's serve at love to finish the match.

By winning $50,000, Nystrom topped $1 million earnings in his tennis career.