Fish, who didn’t seem to be 100% focused, pulled out of the Davis Cup tie against France a couple of weeks ago due to what he called a health issue.
“What happened a couple of weeks ago scared me. Obviously, I’m not over it. I think anyone could see I wasn’t myself.” said Fish after the match.
Russell will play fellow American Ryan Harrison next, who overpowered Igor Kunitsyn of Russia, winning 6-2 6-4.
Second seed John Isner needed three sets on his way to victory as he beat the Argentine qualifier Horacio Zeballos 6-7 (3) 6-4 6-3.
He will meet another American Ryan Sweeting in the last eight. The defending champion Sweeting outplayed Bobby Reynolds in straight sets 6-3 6-2.