The 34-year-old player beat his opponent 5-7 6-3 6-2 in their match yesterday (10.10.21) but it was his use of an underarm serve, which scored an ace, that provoked controversy.
He said: “If you look where [Alvaraz] was standing at the beginning of the match to return the first serve, in comparison… he changed that at the end of the first set.
“He started returning from further back, so it was even harder to get free points on the serve.
“I thought: ‘If he’s going to stand that far back and I’m getting no love from the court and the conditions – the courts are painfully slow here – why not try it and see if I can bring him forward a little bit again?’
“Wasn’t expecting obviously to get an ace out of it.
"He was so far back, he didn’t react.
“I was thinking about using it at times in the third set, but [I was] obviously up in the score, was doing a little bit better, didn’t feel like I needed to.
“Certainly when guys are standing there to return serve now, it’s a smart play.
“It’s the first time I’ve ever done it in a match and I got the ace.
"I served three aces the whole match and one of them was the underarm serve."
Although the crowd didn't seem pleased with Murray's cheeky shot, his friend and fellow tennis star Nick Kyrigos was rather more impressed.
He wrote on his Instagram story: "No way @andymurray."