Novak Djokovic has secured his number one position in the world rankings for the start of next year after a straight-sets victory over Tomas Berdych at the O2 Arena in London this afternoon.

The two-time defending champion wrapped up the 6-2 6-2 win in just 68 minutes for his 30th consecutive indoor victory.

Djokovic broke Berdych in the first game of the first set and from then on it felt almost inevitable that the Serbian would win.

He told reporters: "I'm aware that being number one in the world is one of the biggest challenges a tennis player can have. It's an incredible feeling and I'm very happy."

Djokovic now progresses onto a semi-final with Kei Nishikori on Saturday, who defeated him at the US Open earlier this year.