Novak Djokovic, who has reclaimed top spot in the rankings from Roger Federer, celebrated by defeating Jo-Wilfried Tsonga 7-6 (7/4) 6-3 at the Barclays ATP World Tour Finals at the O2 Arena.

It was certainly not straightforward for the Serb against last year's runner-up, with Tsonga the better player in the first set but unable to convert any of three break points.

Djokovic has been in great form since losing to Andy Murray in the US Open final, winning titles in Beijing and Shanghai, but last week he was beaten in his opening match in Paris after dashing back to Belgrade to visit his ill father.

He admitted victory was important more than just in the context of this tournament, saying: "I had difficult circumstances (last week), some things I had to face.

"That's why this match was very important for me mentally, to break the ice in some way, to return to the shape that I want to be in, the mental state, also to be confident and optimistic on the court and play my game.

"Hopefully I can carry this on to another match against Andy."

The rivalry between the pair, who have been playing each other since their junior days, has increasingly taken centre stage in recent months.

As well as their five-set classic in the US Open final, the pair also played a brilliant three-set final in Shanghai, where Djokovic turned the tables, and the winner tomorrow will have one foot in the semi-finals.

Today Federer faces Janko Tipsarevic and David Ferrer takes on Juan Martin Del Potro in the opening Group B singles clashes, while it will be a big day for England's Jonny Marray.

The Sheffield player will reunite with Freddie Nielsen, the Dane with whom he brilliantly won the Wimbledon doubles title, for their first match against Indian pair Mahesh Bhupathi and Rohan Bopanna.