VALENCIA, Spain (Reuters) - Gael Monfils suffered a setback in his bid to qualify for this month's World Tour finals when the third seed was upset 7-6 3-6 6-4 by unseeded Spaniard Marcel Granollers in the quarter-finals of the Valencia Open on Friday.

The Frenchman must now win next week's Paris Masters, where he has twice finished runner-up, to have a chance of securing one of three remaining places in the eight-man, season-ending competition at London's O2 Arena.

Argentina's Juan Martin Del Potro, the sixth seed, boosted his London hopes by beating unseeded American Sam Querrey 6-2 7-5.

The 2009 U.S. Open champion, who will play Granollers for a place in Sunday's Valencia final, looks to be getting back to something near his best after more than eight months out last year with a wrist injury.

Top seed and defending champion David Ferrer reached the last four when he overcame former world number three Nikolay Davydenko 6-3 6-4.

Spaniard Ferrer, the world number five, has already qualified for the November 20-27 Tour finals along with Novak Djokovic, Rafa Nadal, Andy Murray and Roger Federer.

Ferrer next faces Argentine wildcard Juan Monaco who ousted Spain's Juan Carlos Ferrero 6-3 6-3.

(Writing by Iain Rogers in Madrid, editing by Tony Jimenez)