Roger Federer

John McEnroe sensed the moment. After years at the top of men’s tennis, Bjorn Borg might be vulnerable, “giving in” ever so slightly. So McEnroe, who had beaten Borg at the 1980 US Open and at Wimbledon in 1981, pressed the perceived advantage in their 1981 Open final. He served and attacked with greater urgency, lobbed surprisingly well and closed out Borg in four sets for a third consecutive Open singles title. Months later, when Borg told McEnroe that he was considering retirement at 25, McEnroe realised the significance of his victory.

Tennis may be experiencing a similar moment of change as this year’s US Open unfolds. Roger Federer has won four Opens in a row, but Rafael Nadal’s victories over Federer at the French Open and at Wimbledon, and his move to No. 1 after Federer’s 237-week reign at the top, will provide the backdrop for what McEnroe describes as “some type of gut check” in the men’s draw.

The backdrop for Nadal-Federer is even more intriguing in light of their epic five-set Wimbledon final. Like Borg, who lost four US Open finals, the 22-year-old Nadal has found the Open his most difficult Grand Slam tournament. He has not advanced beyond the quarter finals.

Federer faces a different set of challenges — Nadal’s stroke improvement and surface maturity, and how to regain what Jim Loehr, a noted sports psychologist, describes as the “sense of invincibility” that separates champions from peers.

Physical factors can often dictate a change.

It was Federer, then only 19, who put the dent in Sampras’s almost impenetrable six-year run at the top, with a five-set victory over the 29-year-old Sampras in the fourth round at the 2001 Wimbledon championships. One year later, Sampras was ousted in the second round at Wimbledon by a relatively unknown Swiss pro, George Bastl. Reeling but still dangerous, Sampras walked away for good that fall after winning the US Open for his record 14th Grand Slam title.

Federer turned 27 this month, and his fitness, form and confidence have remained questionable since his bout with mononucleosis last winter.

Rafael Nadal

“Twenty-seven is an age when your body starts talking back to you,” McEnroe said.Loehr, the sports psychologist, said Federer’s failure to complete the comeback from two sets down at Wimbledon had spawned a “sense of doubt,” only months after Federer was being heralded as tennis’s Tiger Woods.

Nadal’s rise has drawn the respect of peers and past champions, especially for his performance at Wimbledon, Federer’s home turf.

“There’s too many good players now,” said Pancho Segura, who had court savvy second to none. He rattled off a list of potential challengers: Andy Murray of Britain; Juan Martin del Potro, a 19-year-old Argentine who has won 19 straight matches; and Ernests Gulbis, a rapidly rising young Latvian player.

“Nadal’s got the best left-handed forehand in the history of the game, and he’s one of the best defensive players on the run,” Segura said. “But the difference is so small between Nadal and the rest. Maybe he lasts one or two years.”

Throughout his ascent, Nadal has remained respectful of his rival. Federer has said he would analyze and assess after the United States Open.

“It’s just all a blur,” he told reporters after losing to Ivo Karlovic in the third round of the Cincinnati Masters at the end of July.

Players who have been to the summit often lack the tolerance to be No. 2 or No. 3, Loehr said. But public perception often overshadows reality, especially when it comes to changes at the top.