When the world's best tennis player erred for the 62nd time, spraying his final groundstroke past the baseline to seal his defeat at Roland Garros on Friday, the victorious Rafael Nadal toppled backward on the dusty, red clay and dissolved in a teenage puddle of laughter and disbelief.

Not only had Nadal just earned his way into the final of the French Open, but he had done so by defeating Roger Federer, his idol and the game's most gifted practitioner, 6-3, 4-6, 6-4, 6-3.

"Is a dream for me," said Nadal, whose biggest victory coincided with his 19th birthday. "Is a dream for me."

As wonderful as the victory was for the Spanish birthday boy, it was even better for tennis -- signaling the emergence of a rare talent whose game and personality ooze charisma; a youngster with the potential to inject new life into a staid sport; and a player who is peerless on clay yet jumping out of his skin to master hard courts and grass.

And though it may not be evident to Federer, Nadal's emergence here at the French Open will also be beneficial to the four-time Grand Slam champion, who has lacked a worthy foil despite promoters' best efforts to cast American Andy Roddick as his rival.

Federer faulted himself for losing Friday's semifinal -- the final hurdle, in all likelihood, to a career Grand Slam. "Started bad and ended bad, basically," Federer said afterward. But he paid telling tribute to Nadal in the face of obvious disappointment.

"I know I can beat him on any surface, which is good to know," Federer said, "because he's going to be a threat in the future."

Nadal's victory set up a left-handed tilt for Sunday's men's final for the first time in the Open era, pitting the fourth-seeded Spaniard against an unseeded Argentine, Mariano Puerta, who outlasted Russia's Nikolay Davydenko, 6-3, 5-7, 2-6, 6-4, 6-4, earlier in the day. It's the second consecutive year an unseeded Argentine has advanced to the French Open final. And while Gaston Gaudio emerged as the unlikely champion in 2004, Puerta is being given only a remote chance against Nadal.

Far from gloating over his semifinal victory, Nadal was humbled by it. His respect for Federer is so great that when he extended a post-match handshake, Nadal's first words were, "I'm sorry."

Recounted Nadal: "He said, 'No, no! You played very well. Good luck for the final. Good luck for your future.' "

If he defeats Puerta on Sunday, Nadal would become the first player to win the French Open in his debut since Mats Wilander in 1982. He would also seize the world's No. 1 ranking, toppling Federer from the perch he has held for 70 consecutive weeks.

It took Nadal just 43 minutes Friday to accomplish what no other player had during this tournament at Roland Garros: win a set from Federer. Nadal did it in unequivocal fashion, breaking Federer's serve four times. "I started very well, it's true, with a very good rhythm," Nadal said through an interpreter. "It's very important to be able to break early."

In the second set Federer found an antidote for the high-bouncing balls that Nadal was slamming at him, slathered in topspin. The Swiss turned the aggressor and charged the net, winning 13 of 15 points. He tagged the lines with flat, blistering forehands, and he connected on 85 percent of his first serves.

But Nadal surged back midway through the third set. With Federer serving at 4-5, Nadal outfoxed the Swiss with a deft drop shot that gave him two break points. Federer fended off one by wrong-footing Nadal, who went sprawling into the clay. He rose more determined than ever, closing the set, 6-4, with a forehand winner to the open court. Nadal leapt into the air and punched the sky, prompting NBC commentator John McEnroe to gush, "He's awesome for this sport!"

Parisian tennis fans had waited a long time for a Federer-Nadal semifinal, which had been hyped since the pairings were unveiled. But the day dawned with drizzly, gray skies, which delayed the start of play 1 hour 40 minutes.

Then Puerta-Davydenko was first up -- the compulsory first course that no one had much of an appetite for. Running a full five sets and nearly 31/2 hours, in which Puerta wore down the wiry Russian over tedious baseline rallies, it pushed back the start of Federer-Nadal to roughly 6:20 p.m.

The gray clouds receded the moment Federer and Nadal stepped on court, and rays of sunlight flooded the court. It was as if the heavens insisted on seeing this match, too. But as play wore on, the matter of light became an issue.

Dusk was approaching by the time the fourth set started at 8:35 p.m. and Federer shared his dismay with the chair umpire after netting a forehand to get broken and fall behind, 4-3.

"I could hardly see the ball in the end," Federer said afterward.

But after conferring with his superiors, the umpire called for play to continue.

Federer would not win another game, letting out a howl of anguish as he missed a backhand to go down, 5-3. From there, a surging Nadal served out the match.

Spain's Rafael Nadal has a 19th birthday to remember, dispatching Roger Federer in the French Open semifinals, 6-3, 4-6, 6-4, 6-3.In Sunday's men's final, fourth-seeded Rafael Nadal will face unseeded Argentine Mariano Puerta, who beat Russia's Nikolay Davydenko, 6-3, 5-7, 2-6, 6-4, 6-4. "Started bad and ended bad, basically," Roger Federer said after losing to Rafael Nadal, of whom John McEnroe said, "He's awesome for this sport!"