Even Roger Federer loses his cool every once in a while.

In the third set of his Australian Open quarter-final thriller against Tennys Sandgren last night, Federer was caught cursing on the court.

But it was the way that he was caught that stunned the 20-time grand slam champion, who was handed a code violation after letting slip a "mixed" swear word in one of the eight languages he speaks.

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The 38-year-old revealed in his post-match press conference that he swore in a mixture of languages, admitting that he was frustrated by the way he received the penalty.

"Back in the day I feel like we could speak to linespeople," Federer said. "Nowadays they tell me, Go speak to the umpire. Okay, I'll go speak to the umpire. I walk across the court because they are not allowed to speak to the players.

"Honestly, to be frustrated at one point for over, I don't know, 15 hours [sic] throughout the match, I think it's normal. I found it a bit tough.

"It's not like I'm known to throw around words and whatever. It's not like the whole stadium heard it either. But no problem, I'll accept it."

Roger Federer questions a line judge during his Australian Open quarter-final against Tennys Sandgren. Photo / AP
Roger Federer questions a line judge during his Australian Open quarter-final against Tennys Sandgren. Photo / AP

Apart from being arguably the best tennis player of all time, Federer is also proficient in several languages – his native tongue is Swiss German, but also speaks fluent German, French and English, as well as some Swedish, Italian, Spanish and Afrikaans.

It turns out the lineswoman who penalised Federer lives in Switzerland, which is how she caught him out.

"It was a mix," Federer admitted when asked if the expletive was in English. "Clearly she (the lineswoman) speaks mixed. Didn't know that."

"Next time I got to check the lines people," he added jokingly.

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Federer came back from 2-1 sets down to take out the quarter-final matchup 6-3, 2-6, 2-6, 7-6, 6-3. He will face Novak Djokovic in the semifinal tomorrow.