Stefanos Tsitispas' father was branded "stupid" by French player Corentin Moutet during a heated exchange at the Ultimate Tennis Showdown (UTS).
Moutet said the Greek world number six's dad - and coach - Apostolos was talking when he was about to serve.
"It's not respectful," the world number 75 told commentators during the match.
"I won't speak about Stefanos, because he's a good guy. But it's just [his] father is stupid, everyone thinks he's stupid."
Tsitsipas did not seem too bothered by the argument, telling Moutet he would "not care" if the Frenchman's coach did the same.
"I had experience with this in the past. I don't want to say more, but I don't see why there should be a problem," said Tsitsipas, who accidentally hit his dad with a racquet during an angry outburst at the ATP Cup earlier this year.
"My father is trying to encourage me before the serve and he's not really interrupting. My opponent makes a big deal out of it."
Tsitsipas went on to win the match and reach the semi-finals, which will also feature France's Richard Gasquet, Italian world number eight Matteo Berrettini and Belgian world number 10 David Goffin.
The behind-closed-doors UTS event is being organised by Serena Williams' coach Patrick Mouratoglou at his academy in southern France.