On-the-mend American Sam Querrey stunned seeded Jo-Wilfried Tsonga, while former U.S. Open champion Juan Martin round match Thursday at the $1.905 million Valencia Open 500 tennis event.

The 116th-ranked Querrey, playing his first tennis since mid-June after having elbow surgery, doused last week's Vienna titlist Tsonga in 7-6 (7-5), 6-2 fashion on the indoor hardcourts at Valencia's City of Arts and Sciences. The world No. 8 Tsonga is trying to qualify for the season-ending ATP World Tour Finals in London later this month.

Up next for Querrey will be a sixth-seeded del Potro, who got past South African and fellow towering slugger Kevin Anderson 6-4, 6-4.

Spanish hero Juan Carlos Ferrero held off rising American Alex Bogomolov Jr. 7-6 (7-3), 4-6, 6-2 in 2 hours, 51 minutes. The 31-year-old former world No. 1 and former French Open champion Ferrero was the Valencia champion back in 2003.

Ferrero will face gritty Argentine Juan Monaco in Friday's quarterfinals.

In other second-round play, Monaco mauled Italian Fabio Fognini 6-3, 6-1 and Spaniard Marcel Granollers cooled off last week's St. Petersburg champion Marin Cilic of Croatia in come-from-behind style, 3-6, 6-3, 7-5. Granollers was last year's Valencia runner-up to fellow Spaniard David Ferrer, this week's top seed. Ferrer also titled here in 2008 and was the runner-up in 2005.

Granollers will encounter third-seeded French star Gael Monfils on Friday.

Former top-five star Nikolay Davydenko advanced without lifting his racquet on Day 4, as France's Nicolas Mahut withdrew from their scheduled bout. Davydenko will face Ferrer in the quarters.

The 2011 Valencia titlist will collect $455,000.