Russian Igor Kunitsyn ended the run of Indian qualifier Somdev Devvarman with a 6-4, 6-1 victory in the quarter-finals of the Washington ATP on Friday.

Somdev had beaten former world no.21 Taylor Dent in the first round and followed it up with a victory over America's Robert Kendrick in the second.

Defending champion Andy Roddick [Images] was upended in the quarter-finals of the ATP event in Washington on Friday, beaten 0-6, 6-2, 6-4 by Serbian world number 93 Viktor Troicki.

Second seed Argentine Juan Martin Del Potro had no such trouble, felling American John Isner 6-4 6-4 to extend his winning streak to 17 matches.

He will now play German fourth seed Tommy Haas, a 7-5, 6-1 winner over unseeded Colombian Alejandro Falla.

Roddick raced through the first set against a clearly nervous Troicki without dropping a game.

However, after missing a chance to go 2-0 up in the second, three-time champion Roddick lost his concentration and allowed Troicki, cleverly varying his pace, to level the match.

Roddick then saved a break point in the opening game of the deciding set before the match was interrupted for more than an hour because of rain.

When the players returned, games stayed on serve until the seventh, when Troicki snatched the break as the American shanked a forehand long.

The Serbian then held his nerve to serve out for victory three games later.

Earlier, Del Potro broke the 6ft-9in Isner's serve once in each set to advance to the last four.

The 19-year-old, who beat Roddick to win the Los Angeles title last weekend after victories in Stuttgart and Kitzbuhel, was too consistent and mobile for last year's runner-up.

However, Del Potro warned that fatigue was setting in after completing his seventh match in nine days.

Former world number two Haas, who had more surgery on his troublesome shoulder earlier this year, found himself 4-1 down to Alejandro Falla of Colombia before bouncing back for a place in the last four.

Haas said he would have to play well to stand a chance against Del Potro.

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