Tennis legend Roger Federer has suffered his worst defeat in 16 years.

The Swiss was taken down 6-3 6-4 in the third round of the Cincinnati Masters by fearless Russian youngster Andrey Rublev.

Outclassed in just over an hour, the match was Federer's quickest loss since 2003 when he fell to Franco Squillari in 54 minutes in Sydney, according to the ATP.

Federer, who has won the tournament more than anyone, made 16 unforced errors throughout the match against his 21-year-old opponent.

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Struggling with his serve, Federer got broken twice in the first set but said it hadn't damped his mood heading into the US Open.

Roger Federer congratulates Andrey Rublev. Photo / Getty
Roger Federer congratulates Andrey Rublev. Photo / Getty

"And there you have it. It set the tone for the match a little bit," Federer said.

"He was super clean — offence, defence, serving well. He didn't give me anything."

Meanwhile, Rublev shed tears of joy following what was the biggest win of his career.

Fist-pumping in celebration after Federer's forehand sailed long to end it, Rublev impressed by winning 81 per cent of his first-serve points.

Andrey Rublev celebrates after defeating Roger Federer. Photo / Getty
Andrey Rublev celebrates after defeating Roger Federer. Photo / Getty

"It is such an amazing feeling when you're playing a legend like Roger and all these people supporting him until the end," he told ESPN in an on-court interview.

"One day I hope I'm going to feel the same.

"Today I was just trying to do my best and was thinking in my head that I need to play every point until the end no matter what."

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The win sets up an all Russian quarter-final with ninth seed Daniil Medvedev.