NEW YORK, AUG. 31 -- Defeat came to Monica Seles and victory to John McEnroe when they least expected it at the U.S. Open today. Third-seeded Seles was so confident of her chances she forgot a small detail: her opponent, the admittedly forgettable Linda Ferrando. McEnroe, aging and unseeded for the first time in 12 years, remembered his game to upset 10th-seeded Andrei Chesnokov and reach the fourth round at the National Tennis Center.

Among the things Seles did not know about Ferrando coming into their match: that the 24-year-old Italian is a serve-and-volleyer; that Ferrando's favorite movie is "Rocky"; and that she has an indefatigable attitude despite her No. 82 ranking. "I don't think she's in the top 50, is she?" Seles said. Ill-prepared and dismissive, the 16-year-old Yugoslavian was upset, 1-6, 6-1, 7-6 (7-3), in a third-round match that contained the most absorbing tennis of the tournament so far.

But for pure electricity, nothing compared with the feeling McEnroe raised as he summoned a game he was not thought capable of these days, defeating Chesnokov in straight and often beautiful sets, 6-3, 7-5, 6-4. McEnroe recovered from deficits of 0-4 in the second set and 1-3 in the third, to those curiously ambivalent roars only he can garner as it became evident he would make the round of 16 and thus is a startling new factor.

The 31-year-old four-time champion fell from No. 4 last season to his current No. 20 largely because of idleness and is in the midst of another in a long series of comebacks. Whether he can succeed remains to be seen, without a Grand Slam title since 1984. But his performance tonight makes him worthy of consideration, as his movement was limber and the ball came off his strings with an old nostalgic effortlessness for 37 winners.

"It was a nice feeling," McEnroe said. " . . . There's still a ways to go, but I can see the potential."

Ferrando now must be the most famous person to come from the seaside resort of Genoa since Christopher Columbus. "Yes, I think so," she said. She is also, it turns out, a savvy veteran and a physically courageous player who survived cramps in both legs and a bad fall to the hard court. Her low, skimming slices and relentless pressing at the net, rushing to it 88 times, took the force out of Seles' lacerating strokes and caused Billie Jean King to remark, "She has played one of the most thoughtful and intelligent matches I have ever seen."

It was the first time in her career Ferrando had appeared on the center court in a Grand Slam tournament, and the most impressive achievement of her career. Asked what she had thought of her chances before the match, she laughed and said, "Not a lot." Actually, she has no other achievements to speak of, never before reaching even the third round of a major. "Right now it is unbelievable," she said.

Strangely it was Seles, the winner of seven titles this year, including the French Open, to rise to No. 3, who was the more prey to nerves in the taut final set, which contained the most absorbing tennis in the tournament thus far. Ferrando held a 2-0 lead and lost it. Seles served for the match at 5-4, but Ferrando broke. Then Ferrando lost three match points with jerky strokes into the net in the 12th game.

As they seesawed, Seles was undermined by growing doubts and the slow, fearful realization that she had entered the match ill-prepared and foolishly overconfident. Perhaps she also realized that switching rackets two weeks before the Open is not a good idea, having gone from Prince to Yonex in a big-money deal.

Ferrando put together a composed tiebreaker, never trailing after the first point. She amassed a 5-3 lead, then got her fourth match point with a low, dexterous forehand half volley to the corner and out of Seles' reach. Seles yielded with a slapped forehand into the net tape on the first ball.

"I just got scared," Seles said. "I wasn't my usual self today. This was not the one I was expecting to lose."

What Seles was expecting was a semifinal meeting with No. 2 Martina Navratilova, and a potential final against No. 1 Steffi Graf. Seles had defeated Graf and Navratilova twice each this season to set up a compelling scenario here and create the most hotly contested season in women's tennis in some time. Each of them possesses a leg of the Grand Slam, Graf winning the Australian Open, Seles the French Open and Navratilova Wimbledon. Whoever can claim her second Slam title of the season may well be No. 1 by the year's end, perhaps not mathematically but certainly in the minds of observers.

Now Seles no longer seems to be a candidate. Moreover, her loss appears to clear Navratilova's way to the final, as the most threatening opponent left in the 33-year-old four-time champion's half of the draw is No. 5 Gabriela Sabatini. The 20-year-old Argentine, who has been supplanted as No. 3 by Seles, advanced to the fourth round with a 6-2, 6-4 victory over Sabine Appelmans. Navratilova progressed this evening, 6-2, 6-2, over Halle Cioffi.

"It's Martina and Steffi," Seles said. "That's what I would bet on."

"I really wanted to play her to avenge myself," said Navratilova, who next meets Manuela Maleeva-Fragniere.

Navratilova shook a verbal finger at Seles for not scouting Ferrando. Seles was so ignorant of her opponent she did not realize she had practiced with Ferrando at a tournament in Chicago this winter. "You do it yourself or you have somebody else do it," Navratilova said. "There's no excuse for it. That's probably taking it for granted."

Seles' loss was second only to top-seeded Stefan Edberg's first-round exit as the biggest of the tournament and came on a none too eventful day otherwise. Third-seeded Ivan Lendl defeated Alex Antonitsch after a lackadaisical start as he lost a 5-3 lead in the first set, 7-6 (7-4), 6-1, 6-2. No. 6 Thomas Muster lost a two-set lead before defeating Jaime Yzaga in an increasingly embattled 6-2, 6-2, 4-6, 5-7, 7-6 (7-3).

Of the six women's seeds who began third-round play, only Seles failed to advance in straight sets. No. 8 Mary Joe Fernandez breezed by Robin White, 6-1, 6-2; No. 9 Maleeva-Fragniere dismissed Dinky Van Rensburg, 6-1, 6-0.

Thanks to her lack of homework, Seles entered the match thinking Ferrando is a baseliner. But Seles' most important piece of newfound knowledge may be this: "Probably not to be too confident when I'm up 6-1," she said. "I don't know myself what happened there."

What happened was this: Ferrando got over a bout with nerves in her first Stadium Court appearance and began charging the net. That was Zina Garrison's tactic in beating Seles at Wimbledon, and it now appears the wisest way to combat the teenager's furious pace and awkward, left-handed angles as she swings with two hands on both sides of her body. Seles is a heavyweight, not likely to be beaten by wild knockout punches. But careful combinations of strokes can cut her to pieces.

"I know this is the only chance to win," Ferrando said. "In the first set I was very emotional to play on the center court and I was afraid to come to the net. Then when I saw that she couldn't pass as well, I had more confidence."

Seles was forced to reach, bend and dig against Ferrando's approach slices. Seles made just two errors in the opening set, but 10 each in the second and third.

"I know that I had her in the match, and then it just went away," Seles said. "At 6-1, I say, 'Okay, finish it off.' And suddenly the whole match turned against me."