For nearly two weeks at the U.S. Open, Roger Federer has been the people’s choice, the man on whom no one would dare wish misfortune. When the tournament welcomed past champions to Arthur Ashe Stadium to celebrate 40 years of Open tennis, Federer — not Andy Roddick, not John McEnroe, not Venus or Serena Williams — received the loudest, and longest, ovation. When Igor Andreev, an underdog if there ever was one, pushed Federer to five sets this week, the crowd enjoyed the show yet wanted no part in an upset. Qualifier Gilles Muller received polite support yesterday as he tried to prolong his quarterfinal match with the four-time defending champion, but it was Federer who received a standing ovation when he sent Muller diving to the court late in the first set tiebreaker.

Federer has always been popular in New York — how could a player with an inimitable style and as much class as any champion in the sport’s history not impress the citizens of this town? There’s something different about the Federer worship this year, however. Instead of rooting for unsurpassed greatness, instead of feeling sophisticated for the mere act of loving the man who performs feats of magic on a tennis court, U.S. Open fans now show sympathy for their struggling hero.

What a change in just a year’s time. Once invincible, Federer now seems vulnerable every time he steps onto the court. Men like Muller don’t seem to fear playing him, don’t seem so convinced that they can’t steal three sets from him. The last four years in Flushing, people have watched Federer and waited for an inevitable flash of brilliance. They still wait, but they also doubt. Will it happen? Will he squander this break point? Will he blow a lead and lose a set? Will he commit a flurry of errors?

It’s a measure of Federer’s greatness that we consider him so vastly diminished for stringing together a season that most other tennis players would consider the best of their careers. By defeating Muller yesterday, 7-6(5), 6-4, 7-6(5), Federer advanced to his 18th consecutive major semifinal, eight more than the previous record set by Ivan Lendl. One more victory, against either Novak Djokovic, last year’s finalist, or Roddick, twice a finalist here, and Federer would play in his third major final of the year. For no other man in history has three major finals qualified as an off year.

Federer, of course, is like no other man. He’s not just chasing titles, he’s chasing history. For Federer to break Pete Sampras’s record of 14 major titles, winning this U.S. Open is not a must, but close to it. It’s unlikely that Federer will age as well as Sampras, since his game relies more on quickness and ground strokes than deadly serves. As Patrick McEnroe, the American Davis Cup captain, said yesterday, the quest for 14 isn’t about to get easier.

“I don’t think he’s playing quite at the level that he was in the last couple of years, but he’s certainly capable of turning it around,” McEnroe said. “This event for him is big, not just for this year and to win one major, but as far as moving forward to next year.”

As much as winning this tournament would boost Federer’s confidence, losing it could wreck it. For four-plus years, Federer has owned this tournament every bit as much as he owned Wimbledon the five years prior to this one. Actually, more so. Federer’s attacking baseline game is better suited to hard courts than grass, and his nimble feet absorb the punishment of the U.S. Open’s hard surfaces better than any other feet in the business. He is the ultimate hard-court player, a perfect combination of grace, power, defense, and quickness. Sampras won his first title at the U.S. Open, but Wimbledon, which he won seven times, was his true home, the place where his serve boomed the loudest and his volleys carved the sharpest angles. Of all the majors, Federer won Wimbledon first, before finding his rightful shrine in Flushing. It’s here that his game has worked better than anywhere else in the world. It’s here that he has dazzled the most, never more so than the afternoon he defeated Lleyton Hewitt in the final and won two sets at love. It was here that Andre Agassi said of Federer: “He’s the best I’ve ever played against.”

Perhaps Federer’s best asset headed into this final weekend is his imposing record at this tournament. He hasn’t lost a match in Flushing since 2003, a string of 32 victories. Rafael Nadal, the new no. 1, looked formidable in his quarterfinal match Wednesday evening, but he has yet to reach a final in Flushing, never mind win the tournament. It took Nadal three trips to the Wimbledon final to dethrone Federer, and he needed the best performance in the history of tennis to do it. If he reaches the final, will he dethrone Federer in New York on his first try? If he does, he would deal Federer a blow more severe than the one he dealt him at the All England Club a few months ago.

Federer isn’t just the people’s favorite in New York because he has struggled this year. He’s loved because tennis fans, and certainly tennis fans in this city, appreciate excellence. For four years here, Federer has redefined the term. This weekend he has a chance to show that he is not done dazzling just yet.

Mr. Perrotta is a senior editor at Tennis magazine. He can be reached at tperrotta@tennismagazine.com.