Roger Federer could be playing professional tennis late into his forties after he made a pinky promise in his pre US Open press conference.

Having taken six months off in 2016, the 36-year-old Swiss tennis player is currently enjoying one of the greatest years of his career.

Federer has failed to win a Grand Slam since 2012 but has incredibly won the Australian Open and Wimbledon in 2017.

He's showing no signs of his age and is considered the clear favourite for the New York held US Open, which begins on Monday.

Andy Murray, Novak Djokovic and Stan Wawrinka are all injured so 31-year-old Rafa Nadal is widely considered to be his main competitor in New York.

Federer was speaking to journalists in a press conference on Saturday and was asked a couple of brilliant questions by a kid.

The budding young tennis player turned journalist asked Federer a question about his nickname which brought laughter from the whole room.

"Switzerland is really cold, right?" he began. "There isn't too much livestock - so why do they call you the GOAT? [Greatest of All-Time]"

"I don't know," Federer responded. "Some fans call me that. I don't call myself that. Yeah, we have a lot of animals in Switzerland, and goats are part of the livestock we have over there, yes."

He was then coerced into agreeing to play professional tennis in eight or nine years time by the hilarious young fan.

"You have been the most consistent player of all time," said the young journalist. "Can you please continue to play for eight, nine years so that I can play you when I go pro?"

Watch Federer's response in the video below.

If both players successfully navigate the first 10 days of the tournament, Federer and Nadal will meet in the semi-final.

"I'd be happy to play him [Nadal] here," said Federer. "I mean, there's like, I don't know—60-plus guys in between us that don't agree we, in our section, should make it to the semis ... But I'd love to play Rafa here in New York."

Nadal, who is the No. 1 seed at the US Open, won his tenth French Open earlier this year - his first Grand Slam in three years.