Defending champion John Isner and Jack Sock each rallied from early deficits today to set up an all-American semifinal in the Atlanta Open.
Isner was not sharp in beating Marinko Matosevic 7-6 (8), 6-4, but took advantage of the Australian's mistakes.
Sock later topped Slovakia's Lukas Lacko 7-6 (6), 6-2.
Germany's Benjamin Becker beat Dutchman Thiemo de Bakker 6-4, 6-2, and will face the winner of today's last match between Israel's Dudi Sela and fourth-seeded Vasek Pospisil of Canada.
Isner, the top seed, dropped his serve in the first game of the match and had to fight off break points in his second and third service games to make the semifinals for a fifth straight year.
He finished the match with his 17th ace, yet won a modest 70 per cent of points on his serve, well below the ATP Tour-leading 93 per cent rate he carried into the quarterfinals.
He also was helped by the timing of Matosevic's errors. The Australian sent an overhand smash well long to enable Isner to break serve for a 4-3 lead in the first set, and then double-faulted to hand Isner set point in the tiebreaker.
Isner then broke Matosevic to take a 4-3 lead in the second set after the Australian double-faulted two more times.
Isner won the event last year after falling in the 2010 and 2011 finals to Mardy Fish and losing in the 2012 semifinals to eventual champion Andy Roddick.
Lacko led 5-3 in the first set against Sock, but the American rallied to force a tiebreaker. He finished off his career-high 18th win of the season with a lob over the Slovakian.
Sock beat Isner in straight sets two weeks ago on grass at Newport, and lost to him in May on clay in Nice.