MEMPHIS, Tenn. -- Denis Istomin took advantage of sluggish play by John Isner to eliminate the third-seeded American 6-4, 6-3 on Tuesday in the U.S. National Indoor Tennis Championships.
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."
Istomin said he could tell Isner was not at his best, including struggling with his normally strong serve, a strength even Isner admitted can normally pull him out of bad play. Istomin said he could tell things were spiraling down for Isner in the final game.
"It was already break, and he didn't have many chances in the second set on my serve," Istomin said. "Maybe he already was going down mentally. Not serving very big. I just returned well today, and that's the way to play against (him)."
Earlier in the day, fourth-seeded Sam Querrey of the United States defeated Alejandro Falla of Columbia 6-3, 6-3 to advance to the tournament's second round.
In the late match Tuesday night, fifth-seeded Kei Nishikori of Japan ousted qualifier Illya Marchenko of Ukraine 6-4, 7-5.
In the women's draw, third-seeded Sabine Lisicki of Germany defeated qualifier Madison Keys of the United States 7-5, 6-3 and seventh-seeded Magdalena Rybarikova of Slovakia beat American Lauren Davis 7-6 (2), 6-2.