The second serve on break point bounced before it reached the net, costing him the game and a third-set lead. As Nadal walked away from the baseline, he smiled ruefully, bit his lower lip and shook his head in disbelief.
"I was a little bit nervous," he later said.
But Nadal's lapses are usually brief, especially at Roland Garros. Soon enough he was celebrating a semifinal victory with a running leap across his favorite court, on his way to a rematch with Robin Soderling in Sunday's final.
Soderling pulled off a stunner when they met in the fourth round last year, and the upset remains Nadal's lone loss in 38 French Open matches.
"He's a very, very dangerous player," Nadal said. "He's one of the best of the world. It will be a difficult match."
Soderling, the 2009 runner-up, returned to the final by beating No. 15-seeded Tomas Berdych 6-3, 3-6, 5-7, 6-3, 6-3. Nadal then swept No. 22 Jurgen Melzer 6-2, 6-3, 7-6 (6).
Soderling has won his past two matches against Nadal, including the shocker at Roland Garros a year ago.
"I know that I can beat him," Soderling said. "I showed it. But every match is a new match."
The No. 2-seeded Nadal has won all 18 sets in this year's tournament, and he's 21-0 on clay in 2010. He seeks to become the second man to win at least five French Open titles. Bjorn Borg won six.
But Nadal's not infallible, as he showed — briefly — in the third set against Melzer. Nadal served for the victory at 5-4 but was broken at love, with the double-fault the coup de grace.
Nadal said his jitters weren't limited to that moment.
"Roland Garros is a bit special, especially for Spanish players," he said. "I was pretty nervous before the tournament. I was nervous during all the tournament. After this match is the day that I am more happy, because I am at the last match ... what I dreamed it to be a long time ago."
While Nadal is 6-2 in Grand Slam finals, Soderling has little experience on such a stage. His only previous major final was last year at Roland Garros, when he lost to Federer in straight sets.
When asked if he's better-equipped now to deal with the occasion, Soderling said, "We'll see on Sunday, but of course it feels better. It's always the most difficult one, playing your first. ... Hopefully I won't be as nervous as I was last year."
A serve that reaches 140 mph and an equally ferocious flat forehand make Soderling dangerous. He's 27-9 this year and will leave Paris with a career-high ranking — fourth if he wins, sixth if he's runner-up.
Against Berdych, five sets of lusty swings came down to the last four games, and Soderling won them all. In the final game, after 3½ hours in the hot sun, he ran sideline to sideline to chase down a ball and rip a forehand winner on the run.
"Greatest shot of the match," said fellow Swede and three-time French Open champion Mats Wilander.
Match point came moments later, and when Berdych pushed a weary backhand wide, Soderling covered his face with his hands as his accomplishment sunk in.
It was only the fifth five-set win in Soderling's career.
Later, Nadal drew chuckles from reporters when he said Soderling has raised his level in recent years not only as a player, but as a person. They had a testy match at Wimbledon in 2007, and Nadal said Soderling is now friendlier with other players.
"He says more times hello," Nadal said. "Yeah, I am speaking serious. I am speaking in the positive way, not in a negative way. I think he was very shy in the beginning."
As for tennis: "His serve is very powerful, both on the first and the second ball," Nadal said. "He's very aggressive from the baseline. Sometimes he will play long, flat shots. So it's very difficult to make him run."
While Soderling's on the rise, Nadal is chasing down shots and hitting his topspin forehand with characteristic flair. There's no evidence of the knee tendinitis that hampered him last year, and he seems intent on reclaiming his crown as the king of clay.
When asked to compare his play now with a year ago, he demurred, mindful that Soderling sent him home early in 2009.
"Impossible to compare," Nadal said, "because last year I was in the swimming pool in Mallorca."