Federer swore loudly in a “mix” of languages during the third set, before he came from two sets down — saving seven match points — to beat 100th-ranked Tennys Sandgren.

The six-time champion Federer, who speaks English, German and French — as well as some Italian, Spanish, Swedish and Afrikaans — was overheard by a line judge, who reported it to the umpire.

“It was a mix (of languages),” Federer said. “Clearly she speaks mixed. Didn’t know that. Next time I got to check the lines people.”

It was a rare flare-up from the normally unflappable Swiss, who said he was frustrated about a groin injury that was hampering his movement and required a medical time-out shortly afterwards.

The 38-year-old, now into his 46th Grand Slam semi-final after recovering to beat Sandgren 6-3, 2-6, 2-6, 7-6 (10/8), 6-3, said the warning was harsh.

“Honestly, to be frustrated at one point throughout the match, I think it’s normal,” Federer said. “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.”