ESPN 2008 US Open

His nephew had just played the best grass-court match of his 22 years, but Toni Nadal was already thinking ahead to the simmering summer season of hard-court tennis.

"Rafael," the coach said while tapping his temple, "needs to play like he is on grass all the time."

This was six weeks ago, and Rafael Nadal had just embarrassed -- humiliated is not too strong a word -- Andy Murray in the Wimbledon quarterfinals, doing so in less than two hours. Nadal had vaulted out of his clay-court, retrieve-mode shell and was surprisingly (and uncharacteristically) aggressive.

Convincing Nadal to play out of his comfort zone on grass, where the low, skidding bounces make defending far more difficult, was an easy sell for Uncle Toni. On hard courts, with truer bounces and a more stable surface that lends itself to better defense -- for opponents, too -- Rafa often reverts to passive-nonaggressive form.

Standing in the hallway between the players' lounge and the interview room, Toni Nadal sighed. He has been working with his nephew for 18 years now.

"He is young still," Toni said. "He has learned how to play on the grass, and make two finals. He will learn this on hard courts."

Four days later Nadal would play in his third Wimbledon final and defeat Roger Federer in a breathtaking five-setter that was widely acclaimed as the greatest match ever. But the question remains: Can Nadal, the new No. 1, close the 2008 deal at the U.S. Open?

Can he play aggressively enough to win on the surface that he is least comfortable? Despite a hard-court title in Toronto and an Olympic gold medal, history says no.

Here are the facts: While Rafa has a combined record of 50-4 at Roland Garros and Wimbledon, his record on the hard courts in Melbourne and New York is a more pedestrian 25-9. The U.S. Open remains the only Grand Slam tournament in which Nadal has yet to reach at least the semifinals. Since he became the No. 2 player in the world, he has been eliminated at the U.S. Open, in order, by James Blake (third round), Mikhail Youzhny (quarterfinals) and, last year, to David Ferrer (fourth round).

Nadal's evolution from clay-court specialist to master of grass has excited comment throughout the world of tennis, from bottom to top. Earlier this month, Great Britain Davis Cup coach Paul Annacone was talking to one of his former athletes.

"I spoke to Pete half an hour ago," Annacone said from a Binghamton, N.Y., Challenger event, where he was watching two young prospects play. "Pete said about Rafa, 'He just knows on grass he's got to be more offensive.'

"Having that clear, committed view -- he hits the ball so heavy -- was the key at Wimbledon. For much of the match, Roger didn't have a lot to say because Rafa was dictating the tempo of the points. On today's hard courts, he might not be quite as decisive as to when to pull the trigger."

Annacone coached Pete Sampras, who holds the men's all-time record of 14 Grand Slam singles titles, for more than seven years.

"Rafa has to find a place where he consistently plays more aggressive than his clay-court counter-punching style," Annacone said. "He played pretty offensive at Toronto and Cincinnati. I think he got tired, more emotionally than physical.

"It's been a long couple of months."

This is the conundrum of Nadal's success. The better he plays, the more matches he wins. The more matches he wins, the more tournaments he wins. After losing to Juan Carlos Ferrero in Rome, Nadal won all five of his matches in Hamburg, then ran off seven more at Roland Garros. He played the Wimbledon warm-up tournament at Queen's Club and won his first five matches on grass, then seven more at the All England Club. A month later, he surfaced on the hard courts in Toronto, ran the table there and, by the time he reached the semifinals at Cincinnati, didn't seem to have much left. Nadal's 1-6, 5-7 loss to Novak Djokovic ended a 32-match winning streak.

Heading into the U.S. Open, Nadal had played 78 matches, 18 more than any other player on the men's side. When he gets physically tired or mentally worn down, Nadal reverts to clay tactics; there were times against Djokovic when he was rallying from seven, eight feet behind the baseline.

Todd Martin reached two Grand Slam finals during his career, the 1994 Australian Open and the 1999 U.S. Open -- both hard courts.

Nadal rarely hits a bad shot on clay and, inevitably, that kind of sustained pressure leads to opponent errors. To err is human, but Martin insists it is crucial to winning on hard courts.

"Pushing the envelope and making errors in points that he wouldn't normally make errors in is OK," Martin said. "It's an inherent challenge. For most players, it doesn't feel good to miss. But, in fact, unforced errors are essential to playing the game better.

"With Rafael on grass, when the ball is not hit well from other side, no matter where it comes from he gets an incredibly good swipe at the ball because on grass even a topspin shot slows down so much. On hard courts, the ball retains its pace through the bounce more than any other surface. That trust in the bounce allows others to ward Rafael off better; they can camp out a little bit."

On clay, Martin says, Nadal beats players without taking risks.

"More than anyone else, he manages to be incredibly aggressive when he is in neutral," Martin explained. "To win on hard courts, he has to take more risks."

A good example of this philosophy is Mardy Fish's win over Roger Federer in the semifinals at Indian Wells, Calif., back in March. Fish, who was recently coached by Martin, knew he had to fail to succeed. He made only 17 of his 50 first serves (34 percent), but faced only one break point. Meanwhile, Fish pressured Federer's serve and wound up converting four of seven break points and won the match in straight sets.

So, can Nadal win the Open? Federer's surprising fade this year -- last week he was taken out by Blake in the Olympic quarterfinals -- seems to leave the door open.

"Sure, Rafa can win the Open," said Annacone, not sounding quite convinced. "Roger is still my favorite. He's going to be unbelievably hungry. I don't know that these guys are good enough to beat Federer in this environment with him not having won a Slam. Rafa still has a good chance, and so does Djokovic. I'd put Murray in that group, then Roddick and Blake a comma behind them.

"Roger has been so good for so long. It's almost like Roger hasn't had to get better for a while. I hate to say this, but Roger, as good as he is, can still get better. The bigger concern: Can he stay motivated to do what needs to be done? After living with that with Pete for eight years, it's very difficult emotionally for Roger to win every tournament every year. The perception is he's set the bar so high that anything other than Roger Federer isn't good enough."

Martin, too, wants to believe in Federer.

"I still like Federer, but Rafael has a great chance," Martin said. "Conditions will also be a big factor. If the court is quick, in Rafael's game, I'm not certain that he can be as offensive with his positioning as he'd need to be to handle that.

"He's made some adjustments with his serve and it's definitely gotten better. That might be the one thread that changed the result from losing Wimbledon in five sets to winning in five.

"Eventually, though, I think it will happen for him on hard courts."

Whether "eventually" will arrive on Sept. 7, when the men's U.S. Open winner hoists the silver trophy, remains to be seen.

Greg Garber is a senior writer for ESPN.com.