Djokovic earlier beat Nadal in their final round robin match [GALLO/GETTY]

Nikolay Davydenko beat Robin Soderling 7-6 (4), 4-6, 6-3 at the O2 Arena to knock defending champion Novak Djokovic out of the ATP World Tour Finals.

Davydenko, Djokovic and Soderling had all finished with two wins in the round robin phase but the third-ranked Serb was eliminated on sets.

Soderling had already secured advancement and finished at the top of Group B.

Earlier Rafael Nadal ran out of steam at the O2 Arena in London when he lost to Novak Djokovic in straight sets in their final match.

Djokovic won 7-6 6-3 meaning the Spaniard leaves the tournament without having won a single set.

"It was (my best performance) as it was my first straight-sets win," Djokovic told Sky Sports.

"Neither me nor Rafa played great tennis today but I gave my best and I'm really happy to get through this match."

Disappointing

Nadal is now on a four-match losing streak after losing to Djokovic in the semi-finals of the Paris Masters this month.

His season has been interrupted by injuries, and Nadal is just 1-9 against top-10 opponents since losing to top-ranked Roger Federer in the Madrid Masters final in May.

Nadal required treatment on his back during the game [GALLO/GETTY]

He lost his opening two service games although his instinctive battling qualities allowed him to break a weary-looking Djokovic twice to take the opening set into a tiebreak.

Some weak baseline errors allowed Djokovic to forge into a 6-2 lead before Nadal won the next three points only to loop a harmless looking forehand into the tramlines.

Nadal needed treatment on his lower back at 1-2 in the second set. He dropped his serve again in the following game with yet another forehand error and Djokovic stayed out in front to claim victory in one hour 57 minutes.

In Group A, world number one Roger Federer and Juan Martin del Potro moved through to the semi-finals.