Sitting in front of Roger Federer while he speaks about the size of Jack Sock’s bum is not what anyone expected from day one of the ATP Finals in London on Sunday.

Yet two decades into his career, Federer is still able to surprise us and that was certainly an interesting curveball.

In the seventh game of the first set during Federer’s opener against Sock, the two opponents came face to face up at the net, with the former having an open shot on his forehand for the pass winner.

Knowing there wasn’t much he could do to cover the net, Sock simply turned around and bent over, and his tactic worked as Federer netted his passing shot attempt.

“It was a big distraction, I’ll tell you that, because it was very big,” Federer said, sending the room into laughter.

“That’s what I should have aimed for. That target was bigger than the down-the-line court that I had. It’s happened sometimes in the past, but not on a big stage like this.”

Sock explained himself in his press conference saying: “I did it more for fun. Probably do it three times a year. No, it’s not a normal tactic.”

Federer claimed a 6-4, 7-6 (4) win over Sock to claim his 50th victory of the season and begin his assault on a seventh ATP Finals title with routine success.

BEST OF FRENEMIES

Meanwhile, a suited-up Rafael Nadal received his year-end world No.1 trophy on centre court after Federer’s win.

Federer was in contention for the year-end top ranking up until a couple of weeks ago, but his withdrawal from the Paris Masters gave way for Nadal to grab it himself.

Even though he would have wanted to be the one receiving that trophy, Federer was happy to stick around and celebrate Nadal’s achievement on Sunday.

“I was supposed to do the Sky interview afterwards, and they wanted to talk to me about the tactical element of the match. The Rafa trophy presentation was going on. They said, ‘Is it okay to wait?’

“I said, ‘Well, yeah, I better wait because this is a big deal. This is a huge thing’.”

What a special kind of rivalry those two share!