Argentine Juan Martin del Potro was forced to do double duty at the Citi Open in Washington, coming through with a pair of victories to advance to the quarterfinals of that hard-court event.

The big-hitting world No. 7, who lost an epic semifinal match at Wimbledon this year against world No. 1 Novak Djokovic, continued his strong run of form by defeating American Ryan Harrison 6-1, 7-5 and Australian Bernard Tomic 6-3, 6-3 on Thursday.

The top seed had to play two matches due to rain on Wednesday.

Del Potro is seeking his second title of the season and looking to tune up his game ahead of the U.S. Open, which gets underway later this month.

Next up for the six-foot, six-inch Argentine will be big-serving South African Kevin Anderson, who eliminated American Mardy Fish 7-6 (7-2), 6-1.

"Even though I had to play two matches, it wasn't that difficult," Del Potro said afterward. "I felt very good physically and I'm also happy with my game."

Third-seeded German Tommy Haas also managed to win two matches on Thursday but was forced to work much harder than Del Potro, rallying past American qualifier Tim Smyczek 3-6, 7-5, 6-3 and then edging Croatian Ivan Dodig 6-4, 4-6, 7-5.

Haas will next take on promising Bulgarian Grigor Dimitrov in the quarterfinals.

Friday's other two quarterfinals will pit Cypriot Marcos Baghdatis against big-serving American John Isner and Australian Marinko Matosevic against Russia's Dmitry Tursunov. EFE