Defending champion John McEnroe withstood a determined, sometimes brilliant challange from 20-year old Czech Ivan Lendl, probably the most improved player in tennis this year, and reached the semifinals of the U.S. Open championships early this morning.

McEnroe, 21, was forced to elevate his aggressive serve-and-volley game to near its peak to prevail, 4-6, 6-3, 6-2, 7-5, in a 2 hour 47 minute match that kept a sell-out crowd of 20,027 spectators at the National Tennis Center enthralled until 15 minutes after midnight.

Lendl, McEnroe's rival for world junior supremacy three years ago, had not lost a set in four matches up to the quarterfinals, and maintained his high level through the first set, schorching McEnroe with blazing service returns and passing shots.

Long-armed and powerful at 6 feet 2, Lendl winds up and goes for broke on practically every return. He had McEnroe clearly preplexed as he belted the ball hard and deep from the base line, hitting winner after outright winner and thumping short balls and following them to the net, early in the match.

Lendl had won his fourth-round match over Harold Solomon by running 18 straight games after Solomon won the first, and for a set and a half he looked as if he might humble McEnroe as well.

One service break decided the first set, Lendl achieving it in the seventh game with a blistering backhand cross-court winner.

He lost only two points in his first three service games in the second set, and had McEnroe on the ropes in the fifth game. Two passing shots and returns that forced a volley error put McEnroe at break point at 30-40, but he won the game with three aces -- all down the center line to Lendl's forehand in the ad court -- and two service winners.

McEnroe got to break point for the first time in the eighth game, and Lendl hit a forehand lob long. That shift of fortune changed the tide of the match.

"I was in trouble early. The key was getting that break in the second," acknowledged McEnroe, who will play Jimmy Connors -- whom he defeated in a testy four-set semifinal at Wimbledon -- on Saturday. Bjorn Bong will play Johan Kriek in the other semifinal.

"I really picked up my game after that; I think I played up my game after that; I think I played solidly the last two sets -- probably the best I've played in the tournament," added McEnroe, whose piercing left-handed serves and razor volleys ultimately cut down Lendl's attacking backcourt game.

Lendl, a flamboyant shotmaker, started missing the passing shots that are his calling card late in the second set, but he is a fighter. After getting down a break in the fourth set, he kept swinging away, caught up at 3-3, and had two break points as McEnroe served at 3-4. The champion escaped that Jam however, broke for 6-5, and served out the match.

McEnroe is always an intense and self-critical competitor, but he was particularly volatile in this match, an indication of the high anxiety Lendl caused him. He took to arguing with officials on several occasions, exchanging words with hecklers in the crowd, stomping his foot like Rumpelstiltskin whne he missed shots, and flailing his racket in exasperation after lunging at Lendl passing shots that whistled by him in maddening clusters.

This way by far the finest match of a 13-hour day of tennis. Otherwise, it was largely a question of legitimate contenders asserting themselves and dismissing pretenders who had overstayed their rightful time in America's premier tournament.

Connors -- the champion of 1974, '76 and '78 who is still the solid No. 3 man in his sport, though he may now be incapable of beating Borg or McEnroe in a major tournament -- put on a good show in battering plucky Eliot Teltscher, 6-1, 3-6, 6-3, 6-0 to reach the semis of the Open for the seventh successive year.

Hana Mandikova, the gifted 18-year-old Czech who has come of age as a major title contender in recent weeks, gained the semis of a Grand Slam tournament for the first time by showing California Barbara Hallquist the exit marked "quarterfinals," 6-2, 6-2.

Andrea Jaeger became the youngest seminifinalist in the tournaments history -- 15 years, three months -- by outsteadying and then out-aggressing Ivanna Madruga of Argentina, 6-1, 6-3, in a tedious match of seemingly endless rallies.

The first game lasted 16 1/2 minutes, and went to deuece seven times. Eight planes roared overhead, taking off at intervals from nearby LaGuardia Airport, before Madruga double-faulted to lose her serve.

Psychologically, that may have been a very important game because Madruga, 19, whose game is based on scampering all over the court, retrieving everything and hitting topspin loopers, was reminded that she was up against someone who could do that too -- and then get into the net and put away a volley.

"I think it made a big difference because she lost a little confidence on her serve," said the athletic Jaeger, who ran off five straight games after Madruga held serve for 1-2 in the first set.

If the players had been equipped with pedometers, they surely would have indicated that they ran miles in this match, trading groundstrokes down the middle, following lazy, wind-blown high-bouncing top-spinners to the backstops, darting in to cover dropshots, then starting whole sequences over again. There was a good deal of purposeful scrambing, some wonderful "gets," but far too much down-the-middle "moonballing" to make the match interesting.

Jaeger is thought to be the youngest seminifinalist ever in any of the Grand Slam tournaments, but that is no big deal to her.

"I'm not going to go out there thinking, 'well, I'm the youngest seminifinalist, so I can lose. I've done what I'm supposed to do, so that's it.' I'm going to go out there and try to do as well as I can," said Jaeger, who plays Mandikova in one of Friday's women's semis. Defending champion Tracy Austin, 17, plays Chris Evert Lloyd, the four-time champion whom she dethroned in last year's final, in the other.

"I'm not saying that I'm just going to win the tournament really easy, I'm not saying that I'm going to get killed," said the spunky Jaeger. "I'm just saying the four players who are left have a good chance to win."

Connors started and finished strongly against Tetscher, a skinny, 21-year old scrapper who, like Connors, prefers to slug the ball hard and harder from the baseline until he can produce a winner or force a short ball to attack en route to the net.

In the middle two sets, however, Connors admitted that he "started to saw a few logs out there." Enough, in fact, to build a cabin. For awhile, it appeared that his backhand approach shot -- usually his most reliable -- was having a love affair with the net.

Connors and Teltscher both play a feisty, crowd-pleasing brand of tennis, digging for every ball as if it were their last, running and gunning with shirttails flying and a full set of sound effects -- primal grunts and desperate gasps that make the game seem like some exquisite form of torture.

For 2 hours and 33 minutes they battled and scrambled, and the daytime crowd of 17,972 spectators loved it. It was an animated, entertaining show. But the tennis was patchy, with far more points decided by error than winners.

Connors said that he "started out hitting the ball unbelievable," and that in the fourth set he hit it "even better," but that was a bit of self-delusion. i

Teltscher made him struggle, and on current form it would seem that Connors is a likely candidate to get carried out, kicking and screaming and fighting, in the semifinals, as he has been in almost every major tournaments since 1978.

The main thing Connors has going for him is his love of the New York crowds and the brash, noisy, passionate atmosphere of the Open, which seems to energize him and appeal to his street fighter instincts.

"Some of the players think that the tournament and the surroundings and the crowds are bad, but I don't think so at all," said Connors, the incorrigible hot dog who, in personality if no longer in stature as a winner, remains the Reggie Jackson of tennis. (Connors probably considers Jackson the Jimmy Connors of baseball.)