Roger Federer's hopes of qualifying for the Barclays ATP World Tour Finals in London were dealt a blow as he suffered a third-round defeat to Gael Monfils at the Shanghai Masters.

The Swiss was playing in his first event since losing to Tommy Robredo in the fourth round of the US Open and Thursday's 6-4 6-7 (5/7) 6-3 defeat represented yet another disappointment in a difficult season.

Federer battled very hard, showing a lot more emotion than usual, but it was not enough to get past the Frenchman.

Things went wrong from the start for the Swiss, who had his serve broken in the first game and went on to lose the opening set.

The second set seemed to be in Monfils’ grasp, but Federer recovered from a break down at 3-4 and then won four points in a row from 3-5 in the tie-break.

He was swiftly behind in the decider, too, and this time he could not mount a comeback.

He had break points to get to 3-4 but they went begging and Monfils served out to love for only his second win in eight matches against Federer.

The 32-year-old is still in a decent position to make the World Tour Finals, especially given Andy Murray’s withdrawal on Wednesday, but it could well go down to the Paris Masters next month.

Jo-Wilfried Tsonga is one of the players with a decent chance to overtake Federer, and he reached the quarter-finals with a 7-6 (7/5) 6-0 victory over Kei Nishikori.

Tsonga next meets Germany's Florian Mayer, who upset third seed David Ferrer 6-4 6-3.

Ferrer has already booked his place at the O2 Arena but he has struggled for form since reaching the French Open final.

Juan Martin del Potro battled with a fever in his second-round match so Tommy Haas’ withdrawal with a back injury ahead of their clash was good timing for the Argentinian.