He said it was scarier than playing a Grand Slam final.
This article originally appeared on BusinessInsider.com.
Roger Federer may have been knocked out of Wimbledon in the quarter finals this year, but he conquered the Swiss Alps with Bear Grylls in the show "Running Wild with Bear Grylls" on the Discovery Channel.
In an interview last week on BBC Radio 2, Grylls said how he had tried to convince Federer to come on the show for a long time, and how it was a unique episode.
"He has often said: 'Look, when I retire I'll do it,'" Grylls said. "But he came back really fired up after winning the Australian Open, he said: 'Come on, let's do it!' I was filming out in the Swiss Alps anyway, so it happened fast and his episode was a really special one."
The pair went up against icy waterfalls, difficult terrain, and fish eyeballs to make it up to the top of a mountain. Then they had what Grylls called an "epic game of ping pong."
"It was on top of the mountain, his hands were so cold, I thought if I'm ever gonna win it it's now," he said. "I've been practising for months on this mini ping pong table, he can hardly hold the bat he's so cold, he's tired, he hasn't eaten forever."
It was first to 11, and Grylls got in the lead at seven-love.
"I thought 'I've got it,'" Grylls said. "The problem is he started to get his eye in and before I knew it was seven-all. And then he took me. There was only ever going to be one winner."
Federer also told Grylls he was way out of his comfort zone, and was a "real scaredy cat" when faced with the climb.
"I said 'What? You're the great Roger Federer, you can't be scared of these mountains,'" Grylls said. "And he goes 'Honestly, I am much more scared standing next to you now holding that coil of rope at the top of this massive cliff and frozen waterfall than I ever am in a Grand Slam final.'"
He added that it just shows, whoever you are, we're all human and we all have fears.
"But great people like him learn to deal with the fear," he said. "Courage is something they often hold quietly, but they keep finding a way to move towards the things that scare them."