The U.S. Open Tennis Championships seldom fail to surprise, and this year is no exception. Nowhere is that more evident than in the 19,000-seat Stadium Court.

The setting for any athletic contest is paramount. Tension is higher on the show courts here than on, say, Court 15. After upsetting Vitas Gerulaitis on the adjoining 7,000-seat Grandstand Court, 16-year-old Aaron Krickstein lost to Yannick Noah in the stadium.

Said Noah after the match: "I knew Aaron would have pressure because he is playing the No. 4 seed and it's the first time he is playing in the stadium."

It was also on the hard green Stadium Court that newly crowned NCAA champion Greg Holmes defeated sixth seed Guillermo Vilas in three sets. But the biggest upset so far was No. 16 Bill Scanlon's ousting of top-seeded John McEnroe in four sets.

Last year's champion, Jimmy Connors, has played all of his singles matches so far on the Stadium Court. For him, the atmosphere there is more suggestive of theater than a stadium. And he is fond of catering to the New York crowds.

"I love New York," said Connors, "and I think they love me. New Yorkers are a little bit nuts and so am I, so we get along fine. When I'm playing my guts out there in front of 20,000 people, well, that's what the game for me is all about."

Connors wouldn't look right on Court 15. He was upset at this year's Wimbledon by Kevin Curren on Court 2 there. Perhaps he'd have won if the match had been played on Centre Court.

After a shaky first set won in a tie breaker, Connors became the first men's semifinalist by defeating 14th-seeded Eliot Teltscher in straight sets Wednesday. He'll now face Scanlon, who defeated Mark Dickson in five sets by winning a tie breaker, 7-4.

With little fanfare, second-seeded Ivan Lendl has been bulldozing his way through the bottom half of the men's draw. Lendl was the only seeded player to reach the quarterfinals without losing a set, and he beat Mats Wilander today to gain the semifinals in straight sets, although the third did require a tie breaker. Of course, easy matches may not leave Lendl match-tough.

Like Lendl, top women's seed Martina Navratilova has coasted. She lost only 12 games in gaining the semifinals and her matches might as well be scored with a clock rather than in games and sets. She has spent less total time on the court in five matches than Johan Kriek and Roscoe Tanner did in their third-round encounter.

Navratilova's semifinal opponent Friday will be Pam Shriver, the fifth seed and Navratilova's doubles partner. Last year on Stadium Court, Navratilova lost to Shriver, but has beaten her the two matches they have played this year.

Said Shriver, chuckling, "I know Martina's game better than anyone else on the tour, but that doesn't help me very much. Then again, it doesn't help anyone else, either."

In the bottom half of the women's draw, Jo Durie of Britain has reached her first Grand Slam semifinal. This is all the more remarkable because two years ago Durie was laid up following back surgery. Seeded 14th, Durie got to the quarterfinals with the loss of only one set and disposed of unseeded Ivanna Madruga-Osses in a windy afternoon match Wednesday.

Durie's opponent, Chris Evert Lloyd, is my pretournament pick to win a seventh U.S. Open title. In the quarterfinals, Evert defeated eighth-seeded Hana Mandlikova. Evert and Mandlikova played in last year's final, which showed Mandlikova's unwillingness to alter a losing strategy. It was sad watching a gifted athlete use futile tactics in front of a packed house and a television audience of millions. The 1983 version was a little different, but the results were the same: Evert won in straight sets.

Third-seeded Andrea Jaeger put on a poor show in her quarterfinal against Shriver. She whined, stared at the linespersons, pouted and all but cried. At one stage, she fumed to the umpire, "You'd better check those line calls real close, because if there are any more bad calls, I'm going to just walk right off!" Twenty minutes later, she was off, having lost in straight sets.

On the outer courts, the men's and women's doubles, the mixed doubles, the junior boys and girls singles and doubles, the 35-and-over men's and women's singles and the senior 45-and-over men's and women's doubles are being played during this second week.

The most fascinating player here is 17-year-old Luke Jensen of Ludington, Mich. From the deuce court, Jensen, who is unseeded, serves right-handed; from ad court, he serves left-handed. He uses a forehand on both sides or switches to a two-handed backhand on his left side. Or is that two-handed backhand really a two-handed forehand hit cross-handedly? He is drawing crowds who stare in disbelief.

Another interesting aspect has been the play of Jimmy Arias, the men's ninth seed. Just 19 years old, Arias fulfilled his prophesied future this year in winning two premier clay-court events, the Italian Open and the U.S. National Clay Courts. Two weeks ago at the ATP Championships, he came within a whisker of beating McEnroe on a cement court exactly like the ones at the Open. Arias is a feared player on any surface. Tonight, he surprised Yannick Noah, 7-6, 4-6, 6-3, 1-6, 7-5, to gain the semifinals against Ivan Lendl.

Connors and Evert thrive on the drama of Stadium Court, where all of the remaining singles will be played. Between them, they have won 10 U.S. Opens. This weekend, they may add two more.