Second seed Jurgen Melzer suffered a shock defeat at the Winston-Salem Open when beaten by Russia's Igor Andreev.
The Russian troubled Melzer with his serve throughout as he won 7-5 6-3.
Australia's former world number Lleyton Hewitt was eliminated in the first round with a 6-4, 7-6 (7/3) loss to Slovenia's Blaz Kavcic.
"He's the biggest name player I've beaten. I just tried to attack a little bit with my forehand and I think I need to do that a little more," Kavcic said.
Hewitt, who has missed the last five weeks while recovering from a foot injury, said: "He doesn't have any obvious weaknesses out there. His serve is a little patchy at times but some of the break-point opportunities I did get he came up with some good first serves."
Eighth seed Marcos Baghdatis capitalised on all five of his break points as he claimed a 7-5 6-1 victory against Ryan Sweeting.
Victory brought to an end a three-match winless streak for the 26-year-old.
Luxembourg's Gilles Muller fired 26 aces in his first-round match against Kei Nishikori, but was still edged out by the Japanese qualifier 1-6 6-4 7-6 (7/5) after two hours and 12 minutes.
Israeli Dudi Sela earned a second-round meeting with American John Isner, the fourth seed, after dismissing Italian Filippo Volandri 6-3 6-2.