Novak Djokovic's 23-match unbeaten run came to an end as he went down 7-5 6-2 to Roger Federer in the ATP World Tour Finals in London.

World number one Novak Djokovic met Roger Federer in the group stages of the ATP World Tour Finals in London .

The Swiss was serving well and appeared to be producing some of his best form to match Djokovic in the early stages of the first set.

He was also having some joy against the World number one's return - Djokovic saved one set point, but Federer took the next one with a sublime half-volley to win the first set 7-5.

The 17-time Grand Slam champion rolling back the years against an opponent who's six years younger than him.

Both players dropped serve early in the second set but then Federer broke again to take a 4-2 lead - even if it was with a little help from the net cord.

The third seed held serve for 5-2 and with the Serb struggling, Federer produced a crosscourt backhand pass to bring up match point.

Djokovic saved the first one, but couldn't save a second as he sent a forehand just wide to give Federer victory 7-5 6-2.

That win sends Federer through to the semi-finals with Djokovic needing to beat Tomas Berdych in his final group game to join him in the last four.