NEW YORK, SEPT. 6 -- Nothing seems unreasonable at the U.S. Open anymore, certainly not an upset, and when ninth-seeded Aaron Krickstein led defending champion Boris Becker by a set and a service break today, it was only in keeping with the rest of the tournament. But Becker changed the heavy atmosphere in the stadium court today as if by force, surviving after four sets to reach the semifinals.

In the past 10 days, the National Tennis Center has been witness to the first-round loss of top-seeded Stefan Edberg to an unknown Soviet, the quarterfinal defeat of third-seeded and eight-time finalist Ivan Lendl by 19-year-old American Pete Sampras and the extraordinary reawakening of four-time champion John McEnroe.

It seemed nothing else could possibly evoke as much comment and confusion, but journeyman Krickstein did his level best to add to it as he extended Becker in their quarterfinal match before yielding, 3-6, 6-3, 6-2, 6-3.

Becker's powerful rally was only slightly less impressive than the belting display of No. 4 Andre Agassi. The 20-year-old from Las Vegas obliterated unseeded Andrei Cherkasov of the Soviet Union, 6-2, 6-2, 6-3, to make his third straight Open semifinal and become Becker's next opponent.

It was significant that Becker and Agassi were immune to the upset fever so many other top-level players have succumbed to, and they must now be considered the co-favorites, as they are the highest remaining seeds.

"My chances?" Becker said with a smile. "Pretty good."

But Agassi had this retort: "I like mine as much as his."

Becker trailed by a break in the second set, 0-2, and was facing another as he served at 0-30 in the third game. But the hulking West German 23-year-old erupted to win the next 14 straight points and five consecutive games. In the fourth set, he recovered from yet another service break, trailing by 3-0, with an act of will that swung the entire match in his favor, and perhaps the tournament.

"I lost the first set and I'm down a break, but I'm not out of the match yet," Becker said. "I try, I keep my mind, and the other guys know that."

It was the first time in six meetings that Krickstein had taken so much as a set from Becker. This from a 23-year-old with a brace on his left knee -- an injury-prone former child star who last season reappeared in the top 10 for the first time since 1985 and is currently ranked ninth. Krickstein's game lacks a single outstanding facet, save for good pace on his groundstrokes and an eye for placement -- and a penchant for long, tiring matches.

For instance, Becker defeated Krickstein in last year's semifinals, 6-4, 6-3, 6-4, but it took him 2 hours 49 minutes to do so, with a series of interminable rallies from the baseline. Krickstein's record in career five-set matches is 20-5, and he was dispirited to have let Becker close out today's match in the possibility-laden fourth set after 2 hours and 36 minutes.

He has now reached the Open semis just once in six attempts, and the moral victory of taking a set off Becker was little consolation.

"It's better than nothing, I guess," he said with a shrug.

Krickstein could have had more because Becker's game was rife with inconsistency, as he made just 59 percent of his first serves and had 30 unforced errors. Without his usual sharp edge, Becker had to resort to exercising his presence.

"He was giving me every opportunity to take a big lead in the match and I didn't do it," Krickstein said. " . . . He's a big match and a big point player. I didn't feel he played his best tennis today, and if I'd played well, I had a chance. But when you come back from so many situations like he's done, and win so many matches, you've got to have all the confidence in the world. He feels he can win matches even when he doesn't play well."

That summed it up. Becker played miserably in the first set, spraying 14 of his errors. Krickstein broke serve in the fourth game, blasting a forehand pass for break point and then accepting the game when Becker launched a backhand wide. Becker broke back, but then Krickstein accepted another gift when Becker lost his serve again in the sixth, this time on a double fault.

Becker wanted to approach the net, but Krickstein's strokes had enough depth to push him backwards time and again. Becker had to kill three set points just to hold serve in the eighth, but that only forestalled things, because Krickstein grabbed the set in the next game, ending it with an ace.

When Becker lost his serve again to start the second set, it looked like a rout might take place. Krickstein punched an inside-out forehand down the line that Becker barely got a racket on, for the break and the 1-0 lead. Becker, looking uninspired and heavy-limbed, just tried to be patient.

Trailing by 0-30 in the third game, Krickstein suddenly lost his feel for the ball, and Becker seized the opportunity to storm the net.

Becker bludgeoned a forehand to the corner to get a break point against Krickstein in the sixth game. Krickstein, driven backwards as Becker charged the net, gave him the game and a 4-2 lead with a desperate backhand crosscourt that caught the net.

"He was playing so well," Becker said. "I just thought, okay, he has to play like that for the next two hours. . . . But he didn't. An hour and a half was enough for him."

Krickstein's inroads suggested Becker still has not approached last year's masterly level of play, as he won both Wimbledon and the Open. But even with his comparatively lackluster showing, Becker is the most prepossessing candidate left in the Open. Unseeded McEnroe and No. 12 Sampras will decide the other semifinal.

"You never count Becker out," Agassi said. "He squeaks a match out, and the next thing you know he's playing the best tennis of his career at the end of the tournament. He comes with it when he needs to."

If Becker does not lift his game for Agassi, however, he may be denied a place in the final. Just this spring Agassi made his first Grand Slam final, losing to Andres Gomez of Ecuador in the French Open. Agassi is stronger than ever, newly shorn of his scraggly, affected beard, and razor sharp.

Although Becker leads their career series by 3-1, they have a genuine rivalry. Agassi won their last encounter, in the semifinals of the U.S. Hardcourts earlier this season, 6-4, 6-1. They played a memorable five-set, three-tiebreaker Davis Cup match in 1989, when Becker rallied from a two-set deficit.

"I'll be out for blood," Agassi said. "Like I'm sure he is."