NEW YORK, SEPT. 5 -- John McEnroe and Pete Sampras did not just win matches today, they made announcements. Every so often two generations meet on an occasion like this, the quarterfinals of the U.S. Open, and the meaning is perfectly clear. McEnroe, 31, defeated David Wheaton as if to state the past is not yet over. Sampras, 19, who just may be the future of American tennis, upset third-seeded Ivan Lendl in five sets to become McEnroe's opponent in the semifinals.

It is one thing to reach the quarters, any scrambling mediocrity can do that with a little luck. But it is quite another to make the semifinals, which McEnroe did for the first time since 1985 with a virtuoso performance against the unseeded 21-year-old Wheaton, 6-1, 6-4, 6-4. This was an event, a career-reawakening on Stadium Court that drew a thousand flashes from cameras. McEnroe, fallen to No. 20 in the world and overlooked, now has a startling opportunity to win a fifth Open title. He lost that 1985 final to Lendl in straight sets and has not won a Grand Slam since the 1984 Open.

"I made a point of not expecting anything going in," McEnroe said. "But I was hoping for a lot."

How sweetly ironic that Sampras was one of those skinny, talented kids Lendl invited to his home in Greenwich, Conn., a couple of years ago to train. Lendl's generosity and interest were repaid this way: The student whaled the master with 24 aces, inflicted his earliest-round defeat in the Open since 1981, and ruined his bid to break the record of eight consecutive final appearances he shared with Bill Tilden. It took 4 hours 5 minutes: 6-4, 7-6 (7-4), 3-6, 4-6, 6-2.

Lendl, 30, has dropped from No. 1 in the world to No. 3 this season. In Sampras he met an upwardly surging star who was born in Potomac, Md., and raised in Rancho Palos Verdes, Calif., and already is ranked No. 12. Lendl was thoroughly defeated in the final set, his serve broken as Sampras drove him backward on match point with a forehand volley and then smashed an angled overhead.

"I just feel it was a matter of time," Sampras said. "I thought I would float around and all of a sudden make my big move. But I didn't think I'd be in the semifinals of the Open at 19. This tops it all. This is number one on my list right now."

The defeat also ruined Lendl's effort to break the record he shared with Tilden. "It's disappointing of course," Lendl said. "It had to end sooner or later." But something more than just Lendl's record may have ended. The once-unassailable top-ranked player is clearly less than what he was after spending most of this season unsuccessfully trying to win Wimbledon, and as he moves into his thirties it is doubtful whether he can fully recover in the face of oncoming players of such force as Sampras.

As they swung back and forth over the match, Lendl's driving ground strokes too often went awry, with 31 errors. Sampras meanwhile had 57, but he displayed velvety ground strokes and quick-volleying hands in addition to his powerful serve, and an appetite for the big point with 38 winners.

"I kept telling myself I was the better player," Lendl said. "I think it was obvious. But I didn't hit enough good shots. He played very well on the important points."

Here are a few of them. Sampras gained set point in the first with a slapped forehand down the line, and then drove a backhand volley away from the racing Lendl like he was slamming a door. In the second set tiebreaker he delivered an ace and three service winners, but the crucial point was a minibreak of Lendl's serve for a 2-1 lead, with a smooth, crouched backhand pass. He never trailed, taking set point with a heavy forehand winner down the line.

But Lendl is nothing if not determined, and it would have been shocking if he had not extended the match to five sets. He stepped on Sampras early and hard, seizing a 3-0 lead in the third and a 4-0 lead in the fourth. Ultimately, however, the fourth may have turned the match back in Sampras's favor. Sampras rallied to win four of the next five games, and threatened to break Lendl's serve a second time in the set as the veteran tried to close it out in the 10th. Lendl summoned two big serves of his own to kill two break points and held to force the fifth, but not confidently.

"That was very important," Sampras said. "If he had won that set 6-1 or 6-2 he could have steamrolled me in the fifth."

Sampras's history in five-setters was not in his favor: He lost a two-sets-to-none lead to Jaime Yzaga in a first-round match here in 1988. But he also had a precedent for upset, knocking off then-defending champion Mats Wilander in the second round last year before losing in the fourth round to Jay Berger.

But Sampras has matured greatly since then, and Lendl was the uncertain one in the final set. Lendl gave up the crucial service break in the fourth game and could not recoup it against Sampras's lacerating deliveries. Sampras held serve in the important seventh game, recovering from a 0-30 deficit and killing a break point. Of the 10 points they played, three were aces and two were service winners by Sampras.

"I was up 30-0 and he hits three aces," Lendl said, with a note of helplessness.

"That was the match right there," Sampras said.

Lendl would not make any long term predictions for either himself or Sampras. "He's very talented, he's very flashy," Lendl said. "But it's very hard to rank him right now." He wrote the defeat off as the price for his obsessive effort to win Wimbledon, the one Grand Slam title he does not possess. He spent three and a half months practicing on grass only to be beaten by Boris Becker in the semifinals, and had been largely idle since, which explained his lack of consistency.

"Unfortunately I'm paying for it," he said.

With Sampras's victory, some larger power seemed to be on McEnroe's side. The Douglaston, N.Y., native reached the semis without having to face an opponent seeded higher than No. 7 Emilio Sanchez, whom he beat over five sets in the fourth round. To make the final he expected his largest obstacle would be old nemesis Lendl. Instead he will have to contend only with Sampras.

"You can't ask for any more than that," McEnroe said.

McEnroe lost to Sampras last month in a three-set match in Toronto, but that was before he had regathered all the elements of his game, and had 39 winners and only nine unforced errors. Tonight McEnroe had variety, touch and sheer presence. The ease of his draw -- aided by the upset of top-ranked and top-seeded Stefan Edberg in the first round -- was bettered only by Wheaton's, who met no seeded opponents in reaching the quarters.

A 6-foot-4 serve-and-volleyer from Lake Minnetonka, Minn., ranked No. 44, Wheaton had a crisp, efficient game that looked pale and simple compared with McEnroe's range of strokes and looming reputation, and he froze.

"He showed flashes of what he was like in the mid-'80s," Wheaton said. "Everyone thought this was the beginning of the end for him. No one really thought he was playing well."

McEnroe broke Wheaton's serve in the fourth game of the opening set with a scathing forehand pass of Wheaton's hanging half volley. He broke him again for a 5-1 lead when he won a rapid net exchange that drove Wheaton to his knees, punching a backhand volley into open court. He then held serve for the set with a resounding ace down the middle. "I wanted to get off to a good start, because I knew it would prey on his mind," McEnroe said.

McEnroe carried that momentum into the start of the second set. Wheaton, playing in his first singles match on Stadium Court, double-faulted twice consecutively in the first game to give McEnroe break point. McEnroe seized it with a combination, a withering forehand return down the line that started Wheaton diving, and then a forehand volley to open court, and he never relinquished that lead.

Wheaton finally swung freely in the third set, enough to accomplish his only break of McEnroe's serve, in the fourth game, when he took advantage of a net cord volley to whip a two-fisted backhand across the court for a winner. He then held for a 4-1 lead.

But McEnroe swept the next five games. He broke Wheaton in the seventh and ninth games, wristing return winners and volleys out of reach. Wheaton's last say in the matter came when he killed two match points as McEnroe served it out in the 10th, with smooth forehand and backhand passes. But McEnroe then delivered a twisting serve to the body Wheaton could only top into the net.

"When you're young it's hard to appreciate the significance of winning it," McEnroe said. "With the disappointment of not doing well here the last couple of years, the frustrations, you appreciate the great moments more."