LONDON: Swede Robin Soderling returned to haunt Rafael Nadal on Monday with a 6-4 6-4 victory as battle commenced in Group B at the ATP World Tour finals.

Rafael Nadal of Spain reacts during his ATP World Tour Finals first round tennis match against Robin Soderling of Sweden in London November 23, 2009. [Agencies]

Nadal's already slim chances of knocking Federer off the top of the world rankings now hang by a thread and defeat against Russia's Nikolay Davydenko in the second of his three round-robin matches on Wednesday would almost certainly mean the Spaniard will not reach the semi-finals.

Davydenko was beaten by Serbia's defending champion Novak Djokovic 3-6 6-4 7-5 in a repeat of last year's final when the season-ending climax was staged in Shanghai.

Djokovic, the form player arriving at London's spectacular O2 Arena after three titles in his last four tournaments, finally subdued the jet-heeled Russian after nearly three hours of high-quality action before swapping shirts, soccer style, at the net.

Soderling, who ended Nadal's 31-match winning streak at the French Open this year and riled him two years ago at Wimbledon, again proved a prickly opponent for the world number two.

Nadal said before the start of the season-ending showpiece that he was not bothered about bridging the 945-point gap between himself and Federer in the rankings.

What will concern him is that right now his shots lack punch and he cannot intimidate opponents the way it once did.

Tendinitis in his knee, an injury which prevented him from defending his Wimbledon crown this year, and the marital problems of his parents, are both mitigating factors but he has not won a title since May and looks strangely vulnerable.

It is all a far cry from the kind of bicep bulging tennis that helped him claim his sixth grand slam title in Australia at the start of the year and left a crestfallen Federer in tears.

Soderling, making his debut at the tournament courtesy of the withdrawal of American Andy Roddick, adopted similar tactics to those he employed to undo Nadal on Parisian clay and was a worthy winner in one hour 38 minutes.

Quite simply he served big and then took every available opportunity to bludgeon his forehand, a stroke that may not be elegant but was once again too hot for Nadal to handle.

"My level right now is not number one, no?" Nadal told reporters. "He is a big player on this surface and if you are not completely calm and playing well in the important moments it's difficult. I'm not far away from my best level but I need more confidence."

Another near sell-out crowd gathered inside the vast, dimly-lit 17,500-capacity arena but the Rafa banners, including one held by three young girls asking for his hand in marriage, were rarely waved as he struggled to get on top of an opponent clearly enjoying being part of the razzamatazz.

Soderling angered Nadal at Wimbledon two years ago when he immitated the Spaniard's habit of tugging at his shorts but he denied any personal grudge against the Mallorcan.

"Sure, it feels better to beat the world number two than the number 200," the 25-year-old Soderling, who lost to Federer in his maiden grand slam final in Paris in June, told reporters.

"I always enjoy beating good players than lower ranked player but, personally, I have nothing against Rafa. We've played a lot of times and we always had good matches."

It was Soderling's backhand that allowed him to break Nadal's opening service game although he did surrender the advantage at 3-1 with a forehand error.

There was a time when unforced errors from the Nadal racket were as rare as British grand slam champions but he offered up two wayward forehands when serving at 4-5 to drop the first set.

Nadal, did break first in the second set thanks to an untimely Soderling double fault but the expected charge did not materialise and Soderling hit back immediately as the Spaniard's forehand again malfunctioned.

Soderling tightened the screws in the 10th game, eking out a match point that Nadal saved after some tentative play from Soderling. However, he earned another chance with a crunching crosscourt winner and this time Nadal ballooned a backhand over the baseline.