Roger Federer reached the semi-finals of the Association of Tennis Professionals (ATP) World Tour Finals with a 7-5, 6-2 victory over Serbian world No.1 Novak Djokovic, whose 38-match indoor winning streak dating back to 2012 came to an end here.
The third-ranked Federer, a six-time champion of this event, has been Djokovic's toughest opponent this season, having now defeated him three times and lost tight matches in the Wimbledon and US Open finals, reports Efe.
Unlike in those championship-round contests, Federer on Tuesday night gave himself a big psychological lift by winning the first set at the O2 arena, thus avoiding having to play catch-up against a younger opponent.
The Swiss, 17-time Major singles champion, also grabbed an early service break in the second, although Djokovic seemed to effect a momentum shift when he broke back in the third game and then held serve to even the second set at 2-2.
Instead, Federer benefited from numerous unforced errors by his opponent to win the final four games of the match and get a confidence-boosting win heading into the weekend.
With the victory, Federer edged ahead of the Serbian 22-21 in their career head-to-head.
Despite the loss, Djokovic is still alive in this eight-man, year-end tournament and can guarantee himself a spot in the semi-finals with a straight-set victory on Thursday over Czech Tomas Berdych.
Berdych lost earlier in the day to Japan's Kei Nishikori 5-7, 6-3, 3-6 in the other Stan Smith Group contest.
Before Tuesday, Djokovic had not dropped an indoor match since losing at the 2012 Paris Masters to American Sam Querrey.
He also lost for the first time since a defeat at the hands of Federer in the final of the Cincinnati Masters in August.
In Wednesday action in the Ilie Nastase Group, Spaniard Rafael Nadal will square off against British world No.2 Andy Murray in the day session before Spaniard David Ferrer takes on Switzerland's Stanislas Wawrinka at night.
