3rd-seeded Isner ousted in first match at Memphis

The victory was Istomin's first win over a top-20 opponent since he defeated Fernando Verdasco of Spain, then ranked 18th, in the London Olympics last year.

"I know it's going to be a difficult match for me, and I need to concentrate and take each chance I had," Istomin said. "I did this. I did good. I broke him in the first set, and then I started to feel better on the returns, and I started to read where he served."

Isner, ranked 16th, struggled through a number of unforced errors and a half-dozen double faults. Istomin of Uzbekistan, ranked 63rd in the world, broke Isner in the seventh game of the first set as Isner hit two shots into the net and sailed a backhand long in the game.

After gaining another break in the fifth game of the second set, Istomin won the set at love on Isner's serve, game point coming on a cross-court passing shot with Isner at the net.

"It is very disappointing." Isner said. "I just went out there and laid an egg really. It's hard to explain. I guess sometimes it happens. You go out there and don't play too well."

Isner said it is representative of his recent play. He said it wasn't necessarily any particular part of his game, adding: "It's everything. Maybe a lot between my ears too. I'm just not playing too well right now. I've been in this situation before. I've just got to work my way out of it. It seems like I am so far away from where I want to be right now."