No matter their loyalties, tennis fans agree on one absolute truth: Roger Federer is the most versatile, most complete, most polished player in the sport, and perhaps ever. His smooth serve, deadly forehand, picturesque backhand, crisp volleys and feather-light footwork make him the ideal model, a man who, win or lose, stands alone for his mastery of the game's techniques and intricacies.  That is, he did stand alone.  At this year's U.S. Open, where Mr. Federer and Rafael Nadal seem fated to play their first Grand Slam final in New York, there's more at stake in the game's greatest rivalry than a trophy. There's a race for supremacy in shotmaking, too. And Mr. Nadal, not long ago considered one-dimensional, is gaining ground. Quickly.  Rafael Nadal, who has won every Grand Slam besides the U.S. Open, will face Russia's Mikhail Youzhny in the semifinals on Saturday.  His transformation from a clay-court scrambler with a fabulous forehand into an all-surface menace who belts aces and punches winning volleys has caught many in tennis off guard.  "I completely underestimated his ability to improve," John McEnroe said.  When Mr. Nadal, 24, first began to garner attention for his clay-court prowess, Mr. McEnroe cast a suspicious glance on the young Spaniard's two-handed backhand. Mr. Nadal hits it with an abbreviated swing that relies largely on his right hand (Mr. Nadal is naturally right-handed, but plays with his left). "That doesn't seem like a shot that's humanly possible on a regular basis," Mr. McEnroe thought. Then he saw it up close. At Wimbledon in 2008, he warmed up Mr. Nadal for his semifinal. Even in practice, Mr. McEnroe was in awe of the stroke's speed and precision. "I was like, 'How did he get that type of power?' " he said. Mr. Nadal won the title that year, beating Mr. Federer in a five-set drama.  The list of Mr. Nadal's improvements over the years is long. Some are subtle, like his default position on the court, which is now much closer to the baseline, rather than four or five feet behind it. Others are readily apparent. His biting slice backhand. His can't-miss overhead. His reflex half volleys, like the one he hit—at the beginning of a 360-degree spin—against Fernando Verdasco in the quarterfinals Thursday evening. Drop volleys, angled volleys and deep volleys are among his tricks, too. "He's one of the best volleyers in the world now," Mr. McEnroe said.  At the U.S. Open, his serve has astounded. When Mr. Nadal won his first French Open title, in 2005, he was content to spin his serve into the court and punish his opponent during the rally that ensued. Now he strikes serves as fast as 135 mph and often ends points before a rally can begin.  So far this tournament, 40% of Mr. Nadal¹s serves have not been put back into play. Mr. Federer¹s percentage is slightly better (43%). Mr. Nadal has 15 more forehand winners than Mr. Federer, 51 more backhand winners and a higher winning percentage at the net (72% compared with 62% for Mr. Federer). To be fair to Mr. Federer, Mr. Nadal has played more overall points, which has given him a little room to pad his numbers. Neither man has lost a set so far. On Saturday, Mr. Nadal plays Mikhail Youzhny in the semifinals while Mr. Federer faces Novak Djokovic.  "Nadal's a kid who's not afraid to work and grind and make changes," said five-time U.S. open champion Jimmy Connors. "I see the young guys play today and I enjoy watching them, but to have an attitude like that, it's a throwback, especially in a time where your success is based on your bank account."  Mr. Nadal's ambition and rapidly evolving game are home grown. Though many tennis champions begin their rise under the tutelage of parents or relatives, most eventually seek expertise outside the family. Andre Agassi's father sent his son to Nick Bollettieri. So did Monica Seles's father. Mr. Federer has received expert coaching since childhood. Mr. Nadal, who never enrolled in a tennis academy, relies on his uncle, Toni.  As boy growing up on the island of Mallorca, Mr. Nadal hit both his forehand and backhand with two hands. Toni Nadal directed him to play left-handed. He put his nephew on poorly kept courts, so he would learn to adapt to bad bounces. He experimented—in fact, he still experiments. At his uncle's urging, Mr. Nadal recently tweaked his service grip, a tiny change that has produced startling results.  "This is a guy who is extremely ritualistic on court," said Jim Courier, the former world No. 1. "I would think it impossible to get him to make even the slightest change midseason."  Mr. McEnroe had a question about Mr. Nadal's development. "Who the hell was Toni Nadal as a coach?" he asked. "It's absolutely brilliant what they've done, the progression. Half of this and you would have said he was an amazing coach."  As versatile as Mr. Nadal has become, to call him Mr. Federer's stroke-for-stroke equal is still blasphemy in some quarters. Mr. Courier said Mr. Federer remained a more complete player. Tracy Austin, the former U.S. Open champion, agreed, though she said Mr. Nadal is "catching up." Roger Rasheed, the current coach of Gaël Monfils and the former coach of Lleyton Hewitt, said he admires Mr. Nadal's intensity, yet he hesitated to put him in Mr. Federer's class. "Federer has more clubs in his bag," he said.  If Messrs. Nadal and Federer do meet in this year's U.S. Open final, the outcome could have much to do with the surface under their feet. Mr. Nadal has beaten Mr. Federer on hard courts before, most recently in the final of the 2009 Australian Open. The U.S. Open's courts, though, play faster. Mr. Nadal can adapt to speed, as he has proven at Wimbledon. There, though, Mr. Nadal's exceptional footwork and balance give him an advantage over most opponents, who have unsure footing on the slippery grass. "On a hard court, everyone has good footing," said Mary Carillo, the CBS and ESPN commentator. "It narrows the gap."  Inside Arthur Ashe Stadium, Mr. Federer's footing is better than good. He has reached the final here six straight years and won the title five times. So far this tournament, he has played sublimely, even in swirling winds. Mr. McEnroe suggested the final could be the toughest test that either man has ever faced.  "This would be a big, big match for our sport," he said. "It would tell us a lot about where Rafa stands and also what Roger's made of." And, maybe, who has the most game in tennis.
Roger Federer once stood alone as the most polished player in tennis. Now, Rafael Nadal rivals Mr. Federer as a complete player.