Top seed Andy Roddick of the United States and defending champion Juan Martin del Potro of Argentina have advanced to the quarterfinals of the Legg Mason Tennis Classic here in Washington. Roddick scored his 500th career victory to reach the round of eight.

Andy Roddick joined an elite fraternity Thursday night becoming only the fourth active player to win 500 matches in his career.  His milestone achievement was at the expense of countryman Sam Querrey, whom Roddick defeated 7-6, 6-4.

After his match, Roddick told the fans that he was amazed to have won so many matches.

"You know that's the equivalent in baseball of something like 500 home runs. And that's kind of a milestone in tennis. So, it's certainly a pleasure to do it here in Washington. And it's a very humbling experience, and who knows, maybe we'll try for 500 more," he said.

Roddick joins Roger Federer of Switzerland, Carlos Moya of Spain and Australian Lleyton Hewitt as the only active players with 500 or more wins.

In the quarterfinals, Roddick plays Ivo Karlovic of Croatia, who ousted Somdev Devvarman of India (7-5, 6-1).

Meanwhile, defending champion Juan Martin del Potro of Argentina made the final eight with a three set win over Lleyton Hewitt (4-6, 6-3, 7-6).  Del Potro plays Sweden's Robin Soderling in the quarterfinals.

Del Potro has played Soderling once before this year - winning in Auckland in January.  The defending champ says he does not have any special strategy for the Swede.

"We played once this year in Auckland, but he is playing much better than that moment," he said. "I don't have any strategy now, maybe before the match.  But I have to be in good serving, good feeling with the ball and with my serve and everything."

Also Thursday, Wimbledon semifinalist Tommy Haas of Germany advanced and will play fourth seed Fernando Gonzalez of Chile.  Tomas Berdych of the Czech Republic made the round of eight and will play American John Isner.

This tournament is one of several key warm-ups for the last major tournament of the year, the U.S. Open, which starts August 31 in New York.