Michael Russell upset top-seeded fellow American Mardy Fish, while John Isner posted a second-round victory on Thursday at the U.S. Men's Clay Court Championship.
The 33-year-old Houston resident Russell breezed by Fish, 6-3, 6-1 in one hour, 14 minutes at River Oaks Country Club.
Fish, the No. 9 player in the world and a 2006 winner of this event, committed five double faults, while Russell was 5-for-8 in break-point chances.
It was Russell's first top-10 win in 13 attempts
Russell will face 19-year-old countryman Ryan Harrison in the quarterfinals, who defeated Russia's Igor Kunitsyn 6-2, 6-4.
Isner dropped his opening set before rallying for a 6-7 (3-7), 6-4, 6-2 win over Argentine Horacio Zeballos that took two hours, 13 minutes.
The 6-foot-9 Isner fired 16 aces and did not drop a serve. He will clash with defending champion Ryan Sweeting, who topped Bobby Reynolds 6-3, 6-2 in an All-American affair.
Fifth-seeded South African Kevin Anderson needed two hours, 42 minutes to subdue American Sam Querrey 7-6 (7-3), 3-6, 7-6 (7-2).
Anderson will tangle with No. 4 seed and Argentine Juan Monaco, who took care of Japan's Tatsuma Ito 6-3, 6-3.
The winner of this $442,500 event will take home $79,880.