Fish bounced out in Houston
In Fish's first match since pulling out of Davis Cup action against France with health problems, the world number nine was clearly out of sorts, allowing Russell to take full advantage.
"What happened a couple of weeks ago scared me," Fish said. "Obviously, I'm not over it. I think anyone could see I wasn't myself."
Qualifier Russell, the world number 136, hit 70 per cent of his first serves and double-faulted just once as he showed no mercy to his compatriot.
"In the beginning, I thought he was playing pretty well and it was neck and neck and then his game definitely deteriorated big time," Russell said. "I just tried to ride the wave."
"I was trying to take advantage of my service game and I took advantage of a lot of his unforced errors. It turned out great.
"He was playing very quick between points. It was unusual. When I'm up a break usually you'll take a little more time but he was kind of rushing through. I wasn't going to stop him. I wanted to keep the momentum going."
Russell now plays Ryan Harrison after the American beat Igor Kunitsyn of Russia 6-2 6-4.
Ryan Sweeting beat Bobby Reynolds 6-3 6-2 in another all-American affair and he will now play second seed John Isner, who needed three sets to get past Argentina's Horacio Zeballos 6-7(3) 6-4 6-2, for a place in the semis.
"I had to dig deep to get through that first set," Anderson said. "I played well in the tiebreaker. I thought I played better than yesterday. I tried to be aggressive and I was able to do that."
Anderson will now meet number four seed Juan Monaco in the last eight after the Argentine registered a regulation 6-3 6-3 win over Tatsuma Ito of Japan.