Former world number one Andy Roddick is in awe at how Roger Federer is still at the top of his game despite being 35 years old.

Federer returned from a six-month layoff earlier this year to upset the odds and triumph in the 2017 Australian Open, marking his first Grand Slam victory since 2012.

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The current world number five further added to his tally, winning titles in Indian Wells, Miami and the Halle Open as he currently maintains an extraordinary 29-2 record in the calendar year so far.

"I don't know how long [he can keep playing for]," Roddick said, as quoted by Tennis World USA. "People are honestly talking about three or four years still.

"It's crazy because it's not even a crazy thought to think that is a realistic possibility. What he's done is amazing.

"My theory was always he can be top 10 as long as he wants to be."

The Swiss ace had suffered a dip in form in recent years, leading Roddick and many others to believe he had fallen behind the likes of Novak Djokovic and Andy Murray.

"I definitely thought he had fallen behind (Novak) Djokovic," Roddick explained. "For three, four, or five years, that match-up wasn't favourable to him."

"The same with (Andy) Murray, and it's always been a tough match against Rafa. So I didn't know if he would win again before the Australian Open this year.

"Then all of a sudden he gets to Australia, the courts are playing a little bit faster, he gets through a couple of tough match-ups, and the guy wins the tournament. He's been far and away the best player in the world for the start of the season. It's a joke."

Federer is now the last remaining favourite at Wimbledon following recent big-name exits as he takes on Tomas Berdych in the semi-final on Friday (14 July).

"He's the best talent, the most versatile talent, left in this tournament by a long-shot with those guys out," Roddick added. "It's just nuts what Roger's doing still."