Nov. 23 (Bloomberg) -- Rafael Nadal lost his cool -- and his opening match in the ATP World Tour Finals.

Robin Soderling defeated the world No. 2 by 6-4, 6-4 in their group-stage contest in London. It was the Swede’s second upset of Nadal this year, after ending the Spaniard’s unbeaten run of four titles at the French Open in May. Prior to Roland Garros, Nadal had won all three matches against Soderling.

“In the important moments, I didn’t have this necessary calm,” Nadal said in a news conference at the O2 arena. “So I had a few mistakes. I played shorter in the important moments. That’s the reason he beat me.”

The last time the Spaniard won a match in London, he walked away from the lawns of the All England Club with his first Wimbledon title 16 months ago. He won the Australian Open, defeating Roger Federer, but then a knee injury, and Soderling, sent him out of the French.

Today, three months after starting his comeback from the knee problem, Nadal lost the first set after he sent a forehand long. A backhand error on match point handed Soderling the victory.

Soderling has turned his career around since ending Nadal’s 31-match win streak at Roland Garros. The 25-year-old right- hander entered the top 10 for the first time last month. He’s playing his first end-of-season finals as a last-minute replacement for the injured former U.S. champion Andy Roddick.

Confident

Making his first major final at the French Open and winning a tournament on clay have boosted the Swede’s confidence.

“I played a lot of big matches this year, and I played well,” Soderling said in a news conference. “I beat a lot of good players. So I’m sure I improved that part of my game,” he added.

“Especially today, I managed to play really well in the important points, which is something you have to do if you want to beat the top guys,” Soderling said.

Nadal hasn’t had an easy time since his first-ever defeat on the clay courts in Paris. His parents separated, while severe tendinitis in both knees sidelined him and forced him to miss the defense of his Wimbledon title. In his absence, Federer won his first French Open to become the first man to win 15 major singles tennis titles by beating Roddick at the All England Club. His victory ended Nadal’s 46-week reign at the top of the ATP World Tour rankings.

“The second half of the year was difficult for me, and probably I lost a little bit this necessary confidence and this necessary calm,” Nadal said. “But I am working really hard to come back at my best level. I’m not far away from my best level.”

No Revenge

Revenge for his loss in Paris was never on his mind.

“I said 100 times, I don’t believe in revenge,” Nadal said. “Every match is a different. I went on court and tried my best, like I’ve done all my life. I didn’t think, for one second, about the Roland Garros match.”

Nadal’s loss has narrowed his chances of ending the season as the No. 1 player. Although only 945 points separate him from top-ranked Federer, the Swiss needs only one more win in the group stages to secure his year-end ranking. An undefeated Tour finals champion may win as much as $1.6 million and gain 1,500 ranking points.

Federer plays local favorite Andy Murray tomorrow, while U.S. Open winner Juan Martin del Potro of Argentina faces Spain’s Fernando Verdasco. Both Federer and Murray won their opening group matches.

The London crowds hadn’t forgotten about Nadal during his long absence from the capital. The 17,500-strong crowd cheered for him after every point, with some fans screaming “Rafa, I love you” or “Vamos, Rafa!”

“The atmosphere was unbelievable,” Nadal said. “I am happy to live this experience here in London. The crowd was really nice with me all the time.”

To contact the reporter on this story: Danielle Rossingh at the O2 arena through the London sports desk at drossingh@bloomberg.net

Last Updated: November 23, 2009 14:10 EST