How much does Roger Federer know about rugby?

He may be the most successful male player in tennis history, but as a boy Roger Federer flirted with a rather different sport.

His mom Lynette is from rugby-mad South Africa, and the 20-time grand slam champion says he used to practice scrums with his dad. Dig into the Federer family archives and there are even photos of a young Roger with an oval ball tucked under his arm.

Today, however, the 38-year-old is the first to admit that he’s the “worst rugby guy,” and few would argue that a career in the middle of a scrum would have been preferable to winning nearly everything there is to win on the tennis court.

The Swiss great was quizzed by CNN’s Christina Macfarlane on rugby’s basics after playing an exhibition in Tokyo, during which he announced his planned participation at the Tokyo 2020 Olympics, and he displayed at least some knowledge of the game.

He didn’t get the number of players in a team quite right, but nailed the length of a match and knew New Zealand’s nickname – the All Blacks. “Come on,” he scoffed.

Describing a scrum, he told Macfarlane: “I like those. I don’t quite understand it – if they couldn’t figure that out in a different, easier way that doesn’t break your spine. It’s hardcore in there.”

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Despite being in Japan at the same time as the Rugby World Cup, Federer confesses most of the tournament has so far passed him by, but he says he is supporting South Africa.

To see more of Roger on rugby, watch the video at the top of the page.