A little over a year ago, as Pete Sampras watched Roger Federer close in on Sampras' record of 14 Grand Slams — the Swiss juggernaut is two away — he thought the two of them should get to know each other.

Federer was therefore invited over to the Sampras house in Beverly Hills for dinner; they hit some tennis balls, talked about the unique pressures of being No. 1, found they shared the driest sense of humor. Over the months that followed, they kept in touch by e-mail and text message — Sampras offering mock-grudging congratulations as Federer racked up more titles — and in winding each other up, developed the idea that maybe they should play one another in earnest.

"I felt there was," Sampras explains, "this short window in which I might just about still be competitive."

No retired champion — not even one as modest as Sampras — ever likes to think he has nothing left in his locker. Federer, who describes Sampras as his idol, was also excited by the idea. This was the man who set the standards that shaped his game, the closest he has to an equal in tennis history.

It would be Nicklaus vs. Woods, Ali facing Joe Louis, Jordan against Kobe. They set up a best-of-three series of exhibition matches in Asia in November. Federer won the series, but not without Sampras, still only 36, overpowering him in the last of the matches on a lightning-fast court in Macau. They will resume their rivalry Monday in the NetJets Showdown presented by Men's Vogue and Rolex at Madison Square Garden (tickets are available at Ticketmaster, and the match will be broadcast live on the Tennis Channel), with Federer taking nothing for granted.

"Pete still has a top-five serve," he says.

There is still a lot of unfinished business between the two players, and the match is sure to stoke the rivalry. Sampras and Federer met only once in tournament play, during the fourth round at Wimbledon in 2001. At the time, Sampras had been undefeated in 31 straight matches at the tournament and had entered that dangerous territory for a legend: He looked like he would never lose. Federer, meanwhile, was known to be immensely gifted but was considered a "choker" — temperamentally suspect, perhaps terminally unable to convert his talent into trophies.

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All of that shifted in five sets on that summer afternoon. Watching from the press box on Centre Court, I had the sense that nothing in the game would be quite the same again.

Tennis players talk about "getting each other's number," by which they mean not only moving up the rankings but something more fundamental. Björn Borg had done it to Jimmy Connors, John McEnroe to Borg, Ivan Lendl to McEnroe. For a decade, no one had gotten Sampras, but that day, Federer, 10 years his junior, showed him the future and made Sampras, a month away from turning 30, look old.

The revelation took a couple of years to sink in, but from then on the rest of the script was already inevitable: Sampras won only one more match at Wimbledon and retired in 2003. Looking back, the American still sees that first loss to Federer as "the turning point in both our careers. Before then, I knew Roger had a really good game, but that day I saw he had a power and a quickness that would make him really special.

"I didn't just hand over the keys there and then, but that was certainly the beginning."

Federer not only took over the American's invincibility, he also borrowed some of his habits. I had breakfast with Federer the morning after his second Wimbledon triumph in 2004; he was staying in London at the house that Sampras had occupied in the years of his seven titles; the keys were well and truly his.

"I always knew I had a shot at No. 1," he said then. "But when I beat Sampras at Wimbledon, I knew I could beat him anywhere in the world."

Even now, Federer suggests that match as one of the "very top moments in my career."

When they meet again at the Garden, they're playing for the history books.

"The thing was, when I quit," Sampras says, "I had nothing left to prove to myself. I never played for the money, I never played for the limelight. I just wanted to see how good I could be."

In the greatest champions, some feelings never quite go away.