Hana Mandlikova, the woman most people believe should have been seeded second in the U.S. Open tennis championships, barely survived her opening-round match tonight.

Mandlikova, whose fifth seeding was the subject of much controversy last week, fought off a match point and got through an excruciating second set tie breaker to beat Mary Lou Piatek, 6-7, 7-6, 6-3.

Piatek, the 18th-ranked woman player in the world, kept Mandlikova pinned on the base line for much of the brisk evening with a fine forehand before tiring in the third set. "The first match here is always the toughest for me," Mandlikova said. "I think I had a very tough draw to play Mary Lou first because she is a very good player. She played good tennis tonight."

Mandlikova was the only seeded player to flirt seriously with disaster on an opening day lengthened considerably by three rain delays totaling nearly 3 hours. Twenty-five of the day's 64 scheduled matches were postponed.

Among the men, top-seeded John McEnroe got a brief shock from Juan Nunez, the 193rd-ranked player in the world, before beating the determined Chilean, 6-7, 6-1, 6-3, 6-2. Third-seeded Ivan Lendl coasted to a 6-2, 6-2, 6-2 victory over Sweden's Hans Simonsson. The fourth seed, Jimmy Connors, easily beat John Lloyd, husband of Connors' former fiance Chris Evert Lloyd, 6-0, 6-0, 6-2. In the featured evening matches, fifth-seeded Jose-Luis Clerc defeated Brad Drewett, 6-4, 6-3, 6-3, and Eliot Teltscher defeated Victor Amaya, 6-2, 6-2, 6-4.

The only seeded men's player to lose was No. 14 Wojtek Fibak of Poland, a 7-6, 6-4, 7-6 victim of South Africa's Kevin Curren.

The best men's match of the day was between nonseeds Stan Smith and John Sadri, Smith gallantly coming back from two sets and a match point down to win, 5-7, 6-7, 7-5, 6-4, 7-6. Because of the rain, it took almost 7 hours to complete the 4 hour 32 minute match.

The women did not follow form as closely.

Third-seeded Tracy Austin was the only seed who had an easy time. She beat Britain's Anne Hobbs, 6-1, 6-2. But 13th-seeded Regina Marsikova and 16th-seeded Dianne Fromholtz were beaten. Marsikova was beaten badly by 33-year-old Rosemary Casals, 6-2, 6-2, and Fromholtz was never in her match, losing to Anne Smith, 6-3, 6-3.

But the match of the day was Mandlikova-Piatek. Piatek, 20, is a rising player who won her first tournament several weeks ago in Richmond. She had Mandlikova in trouble most of the first two sets but could never put her away in a match that saw 19 service breaks in 33 games.

"When she needs a point badly, she always seems to have the shot," Piatek said. "There were a few points where I thought I had passed her or I thought I had hit a winner and she somehow got to the ball. That's what makes her so tough. She can do so many different things."

That was most evident in the second set tie breaker after Mandlikova, leading 6-3, had blown three set points and appeared ready to be upset. At that point, Mandlikova hit a backhand down the line for a winner, then went crosscourt with a forehand for the set. Although Piatek led briefly at 2-0 in the final set, Mandlikova was clearly the stronger player, winning five games in a row to take control of the match for the first time.

"I had trouble getting started in the beginning," Mandlikova said. "I wasn't moving well the first two sets, but part of it was her; she played well. At the big tournaments, I play better every round. Everyone in this room knows I should be seeded second. Even Chris said that. I won the French and reached the final at Wimbledon. I guess I will just have to beat them all."

Aside from Mandlikova's scare, the match that caused the most stir, naturally, was McEnroe's.

He started badly, losing his serve at love in the first game.

Nunez, whose biggest victories have been over Rick Fagel and Bruce Nichols, was hitting all-out on every shot, going for winners and serving well. In the middle of the set he had what was left of the crowd of 19,153 on his side, and McEnroe was beginning to grumble.

Still, McEnroe appeared to have recovered when he broke Nunez to even the set at 5-5, and, at 6-6, take a 5-0 lead in the tie breaker. But, to the surprise of everyone, Nunez hit four straight winners, two off McEnroe's second serves. McEnroe hit a forehand wide to make it 5-5 and the players traded winners to make it 6-6.

Nunez then hit a forehand crosscourt past a stunned McEnroe and a service winner for the set as the crowd began to stir, thinking it might be witnessing the biggest upset in tennis for some time.

That, though, lasted only as long as the 2 hours of rain could delay the inevitable. McEnroe did not play that much better, but Nunez could not maintain his early pace and McEnroe was out of trouble.

Except, of course, with the umpire. McEnroe jousted with Ken Slye of Alexandria, Va., throughout the match. He disputed let calls, was given one delay-of-match warning, got a penalty point for slamming a ball off the scoreboard because he was upset over a bad shot, and drew hoots and laughter when he bowed mockingly to Slye after the umpire ordered him to stop arguing and start playing.

But, after his faltering start, McEnroe never lost serve again, winning his 15th straight match here, dating back to a 1978 semifinal loss to Connors.

"I feel pretty good coming in here; winning Wimbledon helped me," McEnroe said. "It's a little disappointing to have the crowd against you in your home town, but it's not that important."

McEnroe was almost diplomatic in talking about Slye: "He was inconsistent, that's what I didn't like, but what I think doesn't matter."