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.
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.
Nadal's defeat, though, was the talking point of Day Two.
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.
Tendonitis 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.
