LONDON (Reuters) - Swede Robin Soderling claimed his second major upset at the ATP World Tour Finals on Wednesday when he beat world number three Novak Djokovic 7-6 6-1.

Soderling, only competing in the elite year-end showpiece at London's O2 Arena because of the withdrawal of Andy Roddick, backed up his opening Group B victory over Rafael Nadal with another magnificent display to all but seal a semi-final spot.

Djokovic looked disgruntled throughout the one hour 34 minute contest and offered token resistance after losing the first set.

Soderling, this year's French Open runner-up, had never beaten Djokovic in five previous clashes with the Serb and punched the air with delight after his opponent netted a lame volley on match point.

Djokovic, the defending champion after winning the year-ender in Shanghai last year, will need to re-group quickly for his final round-robin match against Nadal on Friday.

Nadal plays his second round-robin match later on Wednesday against Davydenko with both players seeking a first victory.

(Reporting by Martyn Herman; editing by Miles Evans)