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."
Earlier in the tie, world number 17 Karen Khachanov weathered an early storm from Taylor Fritz to win 3-6, 7-5, 6-1 in one hour and 51 minutes.
Fritz, 22, started with an array of blistering groundstrokes and gained a stranglehold when he broke in the fifth game of the second set.
But the world number 32 couldn't consolidate and grew tentative. Khachanov pounced to level the match.
Fritz's spiral continued in a one-sided third set lasting just 29 minutes. Khachanov credited Russia captain Marat Safin for helping his turnaround.
"He just told me to keep going and that I will break him, which is what happened," he said.
Emerging 21-year-old Ruud continued his giant-killing run with a 6-2, 6-2 victory over world number 12 Fabio Fognini, before Italy recovered to down Norway in the earlier Group D tie.
Ruud levelled the tie in just 66 minutes and then backed up alongside Viktor Durasovic in the deciding doubles only to be thwarted by Fognini and Simone Bolelli, who prevailed 6-3, 7-6 (7/3).
Italy join surprise packets Norway with a 1-1 record after being swept by Russia in their opening tie on Friday.
Ruud, whose father Christian is Norway's captain, has led his team's surprising run, starting the season spectacularly after upsetting Fognini and Isner in his first wins over top-20 players.
"Coming out here and beating two top 20 players at the beginning of the year is great for my confidence," he said.
Stefano Travaglia notched his first win of the season in the earlier singles match after comfortably beating Durasovic 6-1, 6-1 in 56 minutes.
The 28-year-old is representing Italy for the first time after replacing world number eight Matteo Berrettini, who withdrew from the tournament with an abdominal injury.
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