Las Vegas: American James Blake won his round-robin match over Juan Martin Del Potro, but was unable to advance to the quarterfinals of the Tennis Channel Open on Thursday. The top-seeded Blake (1-1 in round-robin play) led 6-1, 3-1 when the No 13 seeded Argentine (also 1-1) had to retire because of respiratory distress. Blake, the tournament's defending champion, started strong and won the first five points of the first set. Blake had taken a 3-1 lead in the second set by winning two consecutive games and breaking a 1-1 tie. The 18-year-old Del Potro was the second teenager Blake faced at the Open. The 27-year-old Blake lost to 19-year-old Evgeny Korolev of Russia 6-2, 6-4 in his opening-round match on Tuesday. After losing to Korolev (1-1), Blake had to defeat Del Potro in straight sets and surrender five games or fewer. Because of Del Potro's retirement, Blake could not match the score that Korolev had recorded against him. The unseeded Korolev, who lost to Del Potro 6-3, 6-2 on Monday, advances to the quarterfinals.