One seldom has a realistic chance of beating Roger Federer. Going into his third round contest at the U.S. Open yesterday, the brilliant world no. 1 had won his last 21 matches and 31 straight on hard courts. In Flushing, he had not dropped a set, improving his record this year to 67-3.

But if there ever was a day to show master Federer the door, yesterday was it. Forehands that Federer normally would pulverize sailed just long, or into the net. Topspin backhands that always roll gently off the strings of his racket clanged off the frame. Short balls and high volleys, shots that Federer dispatches with ease, were several times adventurous. After one miss, Federer rapped the side of his head with his racket in disgust. Later, he bounced his racket off the ground.

Across the net from this grand opportunity was Nicolas Kiefer, a 28-year-old German and former top five player who had given the 24-year-old defending champion fits before. Though Kiefer took an early lead and pried loose a set, he ultimately let the match slip through his fingers, losing 6-4, 6-7 (3), 6-3, 6-4. It’s a wonder when he – or for that matter, anybody else – will get such a chance again.

The turning point came in the third set, with each player looking to take a commanding two-sets-to-one lead. By this point, Kiefer had settled into a nice rhythm on his serve, holding with little trouble and swatting aces at regular intervals. Federer, on the other hand, had struggled to hold the opening game of the set and found himself in another hole, 15-40, at 3-3. Spotty all day, the silky smooth Federer emerged, striking a service winner at 120 mph and then, at 30-40, a crosscourt backhand that dipped past Kiefer at the net and landed on the outside line.

“That’s the reason why he’s no. 1, because he makes these shots, these important shots, when he needs,” Kiefer said afterward.

Kiefer, it turns out, hit the shot of the day, a bullet forehand during the fourth set on which he swung downward, as if playing squash. The ball zipped centimeters above the net and skimmed off the baseline, stunning Federer and Kiefer, who watched the replay, smiling wide, on the giant television screen atop Arthur Ashe Stadium.

By that point, however, the match had swung irretrievably in Federer’s favor. Kiefer held a 2-0 lead in the first set, and a 3-0 lead in the second, but had seen both advantages dissipate. After Kiefer failed to break Federer in the third set, he stiffened up on a backhand and then a forehand volley to lose his serve. Federer wasted no time in the fourth set, breaking for a 2-1 lead on a forehand passing shot that landed on the baseline. He finished off the match with another forehand winner on the baseline, leaving Kiefer miffed.

Afterwards, Federer said he did not expect to play his best tennis early in tournaments. As for his next opponent, he said he would prefer to play David Nalbandian, the Argentinean to whom he has lost five of seven career matches (he will get his wish, as Nalbandian won in four sets yesterday).

“I like to play against guys who have beaten me,” Federer said. “Try to get them back.”

Kiefer said he likes Federer’s chances.

“We play on earth, but he’s playing on a different planet,” he said.

Before Federer ended Kiefer’s tournament, no. 3 seed Lleyton Hewitt easily moved into the quarterfinals with a 6-1, 6-4, 6-2 victory over Dominik Hrbaty. Hrbaty, one of the fittest players on tour, had hoped to push Hewitt to five sets like Taylor Dent had done a round earlier, but he committed 49 unforced errors and was never in the match. Known for his incredibly high ball toss, Hrbaty also mystified onlookers this year with his fashion sense, wearing a shirt that displayed the back of his shoulders through two round cutouts.

“I just couldn’t lose to a bloke wearing a shirt like that,” Hewitt said.

In perhaps the most unexpected result of the tournament so far, Jarkko Nieminen, a lefty who has one of the most common surnames in his home of Finland, accomplished a singular feat yesterday when he became the first Finn to reach the quarterfinal of a Grand Slam with a breezy 6-2, 7-6 (6), 6-3 victory over another left-hander, Fernando Verdasco. He faces Hewitt next.

***

Blake or Agassi, Agassi or Blake? The crowd will face a difficult choice tonight in the match of the tournament. John McEnroe, speaking with reporters yesterday afternoon, said to keep an eye on Agassi’s legs and Blake’s nerves.

“The physical part is, is Andre going to be fresh enough, and then the mental part, is James – is it going to be a little bit too overwhelming for him, because he’s playing a guy he really looks up to and respects,” McEnroe said. “A lot of times that’s like being down a break.”