Medvedev blunts Isner's serve as Russia roll on

World number five Daniil Medvedev blunted big-serving John Isner in an almost flawless straight sets victory at the ATP Cup on Sunday.

The US Open finalist hit 24 winners and made just two unforced errors in the 6-3, 6-1 rout in one hour to ensure Russia clinched the tie against the United States ahead of the doubles.

Unbeaten Russia are top of Group D, while winless United States are all but out of contention for the knockout stages in Sydney. World number 19 Isner has struggled in Perth. He lost his opening match to young Norwegian Casper Ruud in three sets.

Medvedev overwhelmed Isner, who consistently misfired his booming serve, and broke the towering American four times.

"I was very scared of his serve, so I knew I had to just wait for my chances. I was really good on return," he said.

Medvedev even out-aced Isner, who for the last four years has been the ATP's year-end ace leader.

"If you asked me before the match, I didn't think it would be possible," he said.

"I was really stable on my serve and there was only one service game where I was tight."