NEW YORK, SEPT. 3 -- An explosion is coming. Any day, perhaps any hour. The U.S. Open has chugged through three days of tennis with barely a whimper from the underdogs. Grand Slam tennis tournaments are not supposed to be this way.

Some might argue that Pat Cash's loss to Peter Lundgren in the wee hours of this morning was a shocker but no one in the locker room seemed to think so. "I bet on Lundgren," Boris Becker said this afternoon. "Cash never really found his rhythm on hard courts this summer."

Since his victory at Wimbledon, Cash has played in three tournaments and has a match record of 2-3. He is still the only male seed to lose and Barbara Potter (No. 15) is still the only woman to lose. Today, Ivan Lendl, Boris Becker, Jimmy Connors, Andres Gomez, John McEnroe, Henri Leconte, Brad Gilbert and Anders Jarryd all advanced to the third round. Only McEnroe, Gilbert and Gomez dropped a set.

One match was a near explosion. Eighth-seeded McEnroe, who always seems to have trouble with night matches here -- more with officials and fans than with opponents -- fought almost everyone in sight, but survived to take a 7-6 (7-3), 2-6, 6-4, 6-2 victory over Richey Reneberg.

The most exciting match of the day took place on Court 16, which sits on the edge of the old World's Fair grounds, and featured Michael Chang, a 15-year-old Californian whose first-round victory made him the youngest male to win a U.S. Open match in nearly 70 years. Chang played tough, but could not eliminate Nduka Odizor of Nigeria, who won, 6-1, 6-2, 6-7 (9-6), 3-6, 6-4.

Playing under an almost full moon, McEnroe had his hands full with Reneberg, a recent Southern Methodist University graduate who gave Lendl a tough time in a third-round match at Wimbledon.

Reneberg broke McEnroe in the first game of the match, forced him to a first set tie breaker, rolled through the second set and seemed to have a chance. But when he needed it most, McEnroe picked up his play and finally pulled out a 7-6 (7-3), 2-6, 6-4, 6-2 victory.

Naturally, it wasn't quite that simple. McEnroe railed at chair umpire Rich Kaufman about a call in the opening game; demanded that Grand Prix Supervisor Ken Farrar do something about a boisterous, bell-ringing crowd on the Grandstand Court (Ecuadorians cheering on Gomez); drew a warning for racket abuse; argued with several spectators, including one boor who threw what appeared to be a towel at him, and, in general had a miserable three hours, finally escaping just before midnight with a well-played though uncomfortable victory.

The women moved on with ease, Steffi Graf, Martina Navratilova, Hana Mandlikova, Helena Sukova, Gabriela Sabatini, Claudia Kohde-Kilsch, Bettina Bunge and Catarina Lindqvist all winning. Only Mandlikova, the 1985 champion here, broke a sweat, taking almost two hours to beat Jo Durie, the lanky Englishwoman, 6-3, 6-7 (13-11), 6-2.

Chang looked overmatched at the start. He was giving away 14 years, five inches and almost 40 pounds to Odizor and the Nigerian quickly won the first two sets and went up a break in the third. But Chang was game. He kept chasing down Odizor's volleys and slowly pulled himself into the match. When he won the third set tie breaker and then took the fourth set, it looked like an upset was possible.

But in the fifth set, Odizor's experience and Chang's lack of five-set experience -- he had never played a fifth set in his life -- showed. The match ended just before the sun disappeared behind the skyline.

The crowd, which stayed until well past dinner time to watch, gave both players a huge ovation as they departed, the implication being that they expect Chang to be back.

In the meantime, the seeds kept trooping off the court declaring themselves relieved to be through the first two rounds.

Perhaps the happiest of the group was Becker, who after coming from two sets down in the first round to beat Tim Wilkison, had a relatively easy time with Jonathan Canter, winning, 6-4, 6-2, 7-6 (7-3).

"I was in a good mood out there today," Becker said. "I was thinking that I probably should be home in Germany or something and I'm still in the tournament. I'm very happy right now."

But he couldn't be happy with his serve. When Becker burst on the scene two years ago with his first win at Wimbledon his serve seemed impregnable. It was so hard, so consistent, so tough to play.

Now, dating back to his second-round loss to Peter Doohan at Wimbledon this year, Becker is struggling with the shot he always took for granted.

"For a long time I almost never practiced my serve," he said. "It was just always there. I worked on the rest of my game, volleying, forehand, backhand, everything else. I didn't need to practice the serve.

"Now, my other shots are much better and that is good. But my serve has been off. I think the problem is in my toss. If I throw the ball in the right place it's an ace every time. But I don't. Maybe I should go home at night and just do this {he threw an imaginary ball into the air} all night."

Today, Becker stopped serving in mid-motion several times just as he did Tuesday night against Wilkison. Up a break in the first set, he double-faulted to let Canter get even. Then, after breaking back, he served three straight aces to end the set.

"That's the way it's been," Becker said. "It just comes and goes. The good thing now is that before I thought to win I had to have my big serve. Without that, I lose every time. If I couldn't serve, I'd go nuts. Now, I know I can win with other shots."

He smiled. "Just imagine how good I will be if I can serve decently, which, hopefully I will do sometime this year."

Becker's play this summer has not been that much better than Cash's. While Cash came out of Wimbledon seemingly haunted by his victory, Becker came out of the Davis Cup match against the United States in July emotionally drained.

"I would go out to play matches and there was no emotion in me," he said. "It was all gone. I just felt very tired in Washington and Montreal. In Cincinnati, I started to feel better.

"Then Tuesday, Tim really brought it out of me. That match was good for me. He practically wanted to box me out there. I needed that. Now I feel pretty good about the rest of the tournament."

On his way back to the locker room Becker stopped off to watch his friend Connors play. He left impressed, "he looks fine to me," as Connors won his 81st Open match, routing Wayne Hearn, 6-3, 6-2, 6-1.

Connors had been concerned about playing two days in a row on his sore right foot but said he felt better today than Wednesday after doctors put some new padding in his shoe to cushion the injury.

Unlike Becker, who is aware of everything going on around him, Connors pays little attention to other players. "I'm too old to worry about anyone else," he said of Cash's loss. "And I'm sure they aren't worried about me."

Cash's exit leaves the top half of the men's draw remarkably stacked. If form continued to hold, the quarterfinal matchups would be Lendl-John McEnroe and Becker-Connors. By contrast, the bottom half is wide open with Stefan Edberg the favorite to reach the final.

The overwhelming favorite in the tournament is still Lendl, who finally lost a game today after his opening round shutout of Barry Moir. Still, Lendl made it look easy against Jean Fleurian, winning, 6-4, 6-2, 6-2. Yet, he wasn't delighted.

"I didn't enjoy the match at all," he said after coming off the shade-filled Grandstand Court. "It was almost impossible to see the ball out there. It was like 'find me if you can' tennis. I'm just glad to have it over."

So was Navratilova, who kept glancing up as the sun began dropping below the rim of the stadium as if she wanted to get her match with Robin White over with before the 5 o'clock shadows showed up. She did just that with a 6-1, 6-3 victory.

She immediately iced her knee, but said it was just a precaution.

"It's nothing," she said. "I feel very good right now. I'm sharp, I'm hitting the ball well. I've worked very hard this year on and off the court. I just haven't been winning as much."

Navratilova is No. 2 in the rankings right now behind Steffi Graf so naturally she is being bombarded with questions about the change.

"I think having Steffi around may keep me in the game longer," she said. "I certainly don't want to go out when I'm No. 2. If she's just better than me, then I can't help it. But right now she's not."

The biggest surprise so far has been the advance of Great Britain's Andrew Castle to the third round. Castle, anointed as a star at Wimbledon two years ago, has gone nowhere fast since then.

He came here as a qualifier, reached the main draw and then stunned David Pate, the No. 19 player in the world, in the first round. Today, he beat Jimmy Brown, 7-6 (8-6), 6-1, 6-3, giving him two wins in a Grand Slam tournament for the first time in his life.

It has been a nice week for Castle. It isn't likely to last much longer, though. His next opponent is Becker, who is not likely to be back in Germany any time soon, at least if form holds. And so far at this tournament, it has been doing just that. TODAY'S FEATURED MATCHES Day (starting at 11 a.m.)

Stadium Court -- Joakim Nystrom (10), Sweden, vs. Ramesh Krishnan, India; Chris Evert (3), Fort Lauderdale, Fla., vs. Niege Dias, Brazil.

Grandstand Court -- Pam Shriver (5), Lutherville, Md., vs. Iwona Kuczynska, Poland; Emilio Sanchez (14), Spain, vs. Todd Witsken, Carmel, Ind.; Miloslav Mecir (5), Czechoslovakia, vs. John Fitzgerald, Australia. NIGHT (starting at 7:30 p.m.)

Stadium Court -- Stefan Edberg (2), Sweden, vs. Dan Goldie, McLean, Va.

Grandstand Court -- Zina Garrison (7), Houston, vs. Terry Phelps, Larchmont, N.Y.; Mats Wilander (3), Sweden, vs. Johan Carlsson, Sweden.