Steve Denton, aided by winds that affected his opponent's serve, upset John McEnroe today and gained the final of the $300,000 ATP Tennis Championships.

Denton, seeded 11th in the 64-player tournament, will play Ivan Lendl, who defeated Jimmy Connors, 6-1, 6-1, tonight for his first career victory in nine tournament matches against the Wimbledon champion. Lendl defeated Connors in an exhibition match in Toronto earlier this year.

Connors scored just 13 points in the first set of the 63-minute match. In the second set, Connors averted a shutout by saving five match points in the sixth game to hold his serve. Lendl took the match on his seventh match point on an error by Connors.

It was the worst loss in Connors' career, which includes two Wimbledon championships and three U.S. Open titles.

For McEnroe, top-seeded and the defending champion, the loss was another in a year-long series. He has won only two tournaments and has been runner-up in four, including Wimbledon, in 1982.

Denton reached the semifinals by upsetting fifth-seeded Vitas Gerulaitis. It is the second time this year Denton has played in a tournament final.

Against Denton, McEnroe, double-faulted five times, four in one game.

"Everything I hit was tentative," said McEnroe, who will defend his U.S. Open title in 10 days. "There is nothing about my game that is not tentative right now . . . I have a lot to worry about right now."

Denton, ranked 19th by the Association of Tennis Professionals, had five aces and nine service winners.

"I think he missed a lot of first serves that he usually doesn't, and that was attributable to the wind," Denton said of McEnroe. "I don't throw my ball up as high (on the serve)."

McEnroe, who was bothered for the second day by a blister on his right foot, also faltered on his shot selection. Asked about one particular shot, he said:

"That was a stupid shot. It's not one to try in a tie breaker with the wind blowing. But then I've been trying stupid shots all week."

In the first set, McEnroe made 48 percent of his first serves and committed four double faults. In the tie breaker, McEnroe made just one first serve in seven attempts.

On the sixth set point, McEnroe had to stay on the base line after missing a first serve. Denton then put away an easy overhead for the set.

In the second set, McEnroe made 46 percent of his first serves and was broken for the only time in the ninth game.

Denton broke McEnroe at 30 in the sixth game of the opening set, winning three of his points on McEnroe's errors.

In the final set, on his fourth break point in the ninth game, Denton's backhand return went past McEnroe and then, held up by the wind, dropped safely into the court.