Atlanta - No 1 seed John Isner beat Ruben Bemelmans of Belgium 4-6, 6-3, 6-4 in the second round of the Atlanta Open on Thursday.
Two-time defending champion Mardy Fish of the United States, the No. 2 seed, withdrew from the tournament with a right ankle injury. Fish was leading Gilles Muller of Luxembourg 6-4, 3-2 when he pulled out.
Isner will play 19-year-old fellow-American Jack Sock, a 7-6 (5), 7-6 (5) winner over Steve Johnson in a matchup of wild cards.
Muller will face Australia's Matthew Ebden, who beat James Blake 6-7 (6), 6-4, 6-4.
In other quarterfinals Friday, fourth-seeded Andy Roddick faces Michael Russell and No 3 seed Kei Nishikori of Japan plays countryman Go Soeda, the No 8 seed.