Jose-Luis Clerc, a man many think capable of winning this tournament, had to overcome a scheduling snafu and a determined Mel Purcell today to advance to the fourth round of the U.S. Open tennis championships.

Clerc's five-set victory, after he lost the second and third sets at love, was the closest any of the top seeds came to serious trouble at the National Tennis Center today. Form has continued to hold through three rounds.

John McEnroe, cheered as he came onto the court, easily defeated Pat DuPre, 6-3, 6-2, 6-3. Vitas Gerulaitis, playing today after a night match Friday, defeated Harold Solomon, 6-3, 6-2, 6-3. Next opponent for Clerc will be Bruce Manson, who upset 11th-seeded Peter McNamara today, 4-6, 6-3, 6-4, 6-2. Gerulaitis next meets Ivan Lendl.Seventh-seeded Gene Mayer beat Dick Stockton, 6-4, 6-4, 6-1, in a night match.

The only change in the pattern of the first week has been the weather. The sun, absent for four days, warmed the stadium court throughout the day today. There was also heat from another source. A trash dumpster caught fire outside the grandstand court where third-seeded Lendl was playing Mark Vines.

As acrid smoke wafted onto the court, Lendl angrily demanded a delay, put on his jacket and stalked off the court to a chorus of boos. After a 20-minute delay, the players returned, only to have the smoke do so, also. That caused a second delay. When the smoke finally cleared, Lendl survived a poor second set to win, 6-3, 4-6, 6-2, 6-3. Then he ducked the postmatch interviews.

There was one upset, a minor one, but it came in dramatic fashion.

While Chris Evert Lloyd and Tracy Austin were winning easily for the third straight time on the stadium and grandstand courts, ninth-seeded Virginia Ruzici found herself in a three-set struggle with Duk Hee Lee, a 28-year-old Korean who has been playing the pro circuit for two years.

Ruzici looked safe serving for the match at 5-3, but in the next two games Lee fought off five match points, including one that had to be replayed after a line judge changed his call. That call, which came with Ruzici standing at the net with her hand extended, apparently unnerved the Romanian. She lost her serve in the 11th game and was upset, 6-1, 4-6, 7-5.

Twelfth-seeded Bettina Bunge, playing on the same court that brought Ruzici to grief, got past Leigh Ann Thompson, 2-6, 7-6, 6-3, to set up a round-of-16 meeting with Evert, a 6-3, 6-0 winner over Alycia Moulton today. Austin had a brief scare, losing the first three games to 17-year-old Pam Casale before winning the next 12 in a row to win, 6-3, 6-0.

Eighth-seeded Pam Shriver whipped Renata Tomanova of Czechoslovakia, 6-3, 6-2.

But it was Clerc who drew the raves today for courage, winning a 3 1/2-hour match that began at 11 a.m. after finishing a four-hour, five-set match at 6:30 last night.

Before Clerc even finished his second-round match with Tim Wilkison, a Saturday schedule had been handed out that had Clerc playing Purcell at 11 a.m. When Clerc and his coach, Patricio Rodriguez, saw the schedule they asked the committee to at least make Clerc the second match afer Austin-Casale.

But the Women's Tennis Association had recommended Austin-Casale to CBS as the women's match most suitable for TV today. "They told us there was nothing they could do," Clerc said. "I think it was a ridiculous thing."

It looked as if it was going to be a fatal thing for Clerc, winner of four straight tournaments on clay this summer, when Purcell blanked him in the second and third sets. Clerc had won the first set, 6-3, then proceeded to go into a near comatose state as Purcell, who seems able to chase down any shot, hit constant winners.

"I was trying to come in a lot because I was tired and I thought if I had to rally from the back court, I would lose," Clerc said. "After the third set, I told myself I just have to run for every shot. In the fifth, I felt better again."

Purcell didn't. The 22-year-old from Murray, Ky., knew he had Clerc in trouble, but let him get away. Serving at 4-all in the fourth, Purcell had a point fro 5-4 but looped a forehand long. That seemed to affect his confidence. He began pushing the ball and that was all Clerc needed. Clerc began blasting his ground strokes again, avoiding the net at all costs. A forehand down the line got him the break and then he held serve to even the match.

That drained Purcell, who quickly went down two breaks in the fifth set and never could recover, finally losing, 6-3, 0-6, 0-6, 6-4, 6-3.

"He just overtook me mentally," Purcell said. "I just couldn't do it on the big points. He killed me that way. His experience this summer helped him a lot. I feel like crying but I'm not going to. He's just a real good player."

Evert and Austin had none of the problems Clerc experienced. Both pronounced themselves very pleased with their play. "I think I'm playing as well as I played last year at the Open," Evert said. "Maybe better."

Austin, who said that having to wait for the end of the Purcell-Clerc match affected her concentration early, was happy to have to struggle slightly. "I think it will be good for me to have been behind," Austin said. "The first three games, I wasn't hitting the ball; I was doing nothing. The rest of the way, I was much better."

With Andrea Jaeger out of the tournament, Austin's half of the draw should allow her to advance to the final. Evert, Hana Mandikova, Martina Navratilova and Shriver are all in the top half of the draw.