Swede Robin Soderling returned to haunt Rafael Nadal on Monday with a 6-4 6-4 victory in their opening Group B round-robin clash at the ATP World Tour Finals. Soderling, who ended Nadal's 31-match winning streak at the French Open this year, again proved a prickly opponent for the Spaniard whose hopes of knocking Roger Federer off the top of the world rankings here are now fading fast. Nadal said before the start of the season-ending showpiece at London's O2 Centre that he was not bothered about the 945-point gap he needs to bridge to claim the year-end No.1 ranking. What will concern him is that right now his game fails to imtimidate 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 No. 1, 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 razzmattaz. Soderling upset Nadal at Wimbledon two years ago when he imitated 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 No. 2 than the No. 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 materialize and Soderling hit back immediately as the Spaniard's forehand again malfunctioned. Soderling tightened the screws in the 10th game, eking out a matchpoint 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. On Sunday, Roger Federer blazed past Fernando Verdasco 4-6, 7-5, 6-1 and join Murray at the top of Group A in the season-ending showpiece. World No. 1 Federer was top billing for the evening session at the spectacular 02 Arena but lost his opening service game to love and sprayed 15 errors in the first set as Verdasco threatened to cause a shock. He then survived a crisis at 5-5, 0-30 in the second set, breathing a sigh of relief as the left-handed Verdasco went agonisingly wide with a forehand – a near-miss confirmed by a Hawkeye replay accompanied by thumping heartbeat sound effect, which was just part of the razzmatazz enjoyed by the 17,500 crowd. Crisis over, 15-time grand slam champion Federer broke serve for the first time in the match in the following game to take the set and then raced away to victory. “I was down a set, and only the second set was I able to sort of get the ball into play, find my range, find my rhythm,” Federer, a four-times winner of the tournament, told reporters. Bryans beaten American duo Bob Bryan and Mike Bryan saw their hopes of ending the year as the world's best doubles team fading away as they lost their opener 6-4, 6-4 to Max Mirnyi and Andy Ram. Top seeds Daniel Nestor and Nenad Zimonjic suffered a shock defeat in the doubles when they were beaten by Poles Mariusz Fyrstenberg and Marcin Matkowski 6-4, 6-4 in their opening Group A round