Halfway up a 3,875-foot mountain, in the middle of a ski resort, John McEnroe, Ivan Lendl and Jimmy Connors are preparing for the U.S. Open.

They have come here, to the Volvo International Tennis Tournament, to play for $315,000 and a new car. More important, though, they have come to begin honing their skills on the hard court surface that they will be playing on two weeks from now in Flushing, N.Y.

Today, on a hot, cloudless summer afternoon, the Big Three cruised into the semifinals of the tournament with routine straight-set victories. They will be joined Saturday by Robert Seguso, who is enjoying one of those magic weeks that players ranked No. 142 in the world aren't supposed to have.

Seguso, who flew here Monday from Hamburg, after playing doubles for the U.S. Davis Cup team over the weekend, began the tournament by upsetting fourth-seeded Johan Kriek. Today, he whizzed through sixth-seeded Scott Davis (the No. 17 player in the world), 6-2, 7-6 (7-3), to win the right to play McEnroe, an easy 6-2, 6-3 winner over Paul Annacone.

"I feel I can play with these guys when I get a chance," said Seguso. "The nicest thing about all this is it ought to improve my ranking enough that I won't have to get up at 7 a.m. on Saturday to play qualifiers any more."

In fact, Seguso, known for his doubles play with partner Ken Flach, has made giant strides this summer. He reached the fourth round at Wimbledon as a qualifier and improved his ranking from 224 to 145 in the process. His play here should move him into the 50s.

The other three semifinalists don't think about numbers that large unless there are dollar signs in front of them. McEnroe and Seguso will be playing for the first time, but Connors and Lendl will play for the 24th. Lendl has won their last five meetings.

Today, both men breezed against players who might have been capable of challenging them. Lendl beat Brad Gilbert -- his doubles partner here -- 6-3, 6-3, in a match that seemed interminable because Lendl fidgets so long between points that darkness can become a factor in a two-set match.

The Old Man -- Connors will be 33 in 24 days -- showed some of his old stuff today in beating Tim Mayotte, 6-4, 6-0. On this surface, Mayotte should be able to beat Connors. But Connors, zinging returns to every corner, reeled off the last nine games.

"When he gets rolling with his returns it's very hard to find a way to stop him," Mayotte said. "I felt great going into the match but when he's good, he's as good as he's ever been."

Connors' problem this year has been being good two days in a row against good players. He admitted today that he has been in "a little slump," but was pleased by his ability to dominate Mayotte.

"I don't know what happened," Connors said. "For the first eight games, he played about as well as I've seen him play. Then, I don't know, he just went off. But I did hit every return today. That makes it hard for him."

Connors should get an idea of where he is Saturday. Lendl destroyed him on clay at the French Open two months ago, but this is the surface on which Connors won two straight U.S. Open finals against Lendl.

The scene, however, is a lot different than the U.S. Tennis Center. The stadium here is a temporary 10,000-seat stadium built only for the week of this tournament. The mountains and trees surrounding it bear little resemblance to the flight pattern of LaGuardia Airport.

And yet, the names, McEnroe, Lendl, Connors, are the same as the ones likely to draw attention at Flushing. Seguso is the interloper. But that makes it a little more fun here on the mountain.