Nick Kyrgios and Boris Becker traded insults on social media on Tuesday following the Australian's criticism of Alexander Zverev after a video appeared to show the German dancing in a crowded club despite the Covid-19 pandemic.
The video was posted on Instagram by German designer Philipp Plein and later deleted. There was no indication of when the video was taken.
Zverev said last week he would self-isolate after playing in Novak Djokovic's ill-fated Adria Tour exhibition tournament in Serbia and Croatia, which had to be abandoned after numerous players tested positive for Covid-19.
Kyrgios on Monday called Zverev, the current men's world number seven, "selfish", prompting a rebuke from commentator and former six-time Grand Slam champion Becker.
"Don't like no #rats! Anybody telling off fellow sportsman/woman is no friend of mine! Look yourself in the mirror and think your better than us... @NickKyrgios," Becker said on Twitter.
Kyrgios was prompt in his reply: "For goodness sake Boris, I'm not competing or trying to throw anyone under the bus. It's a global pandemic and if someone is as idiotic as Alex to do what he has done, I'll call him out for it. Simple."
Zverev, 23, took part in Djokovic's event, which saw crowds packed into stands in Belgrade while players hugged at the net and played basketball in Croatia. The German player said he and his team had tested negative.
"We all live in the pandemic called #Covid_19! It's terrible and it killed to many lives...we should protect our families/loved ones and follow the guidelines but still don't like #rats @NickKyrgios," Becker continued.
Kyrgios had previously criticised the organisers of the Balkan event and hit out at governing bodies for their plans to restart professional tennis during the pandemic.
"Rats? For holding someone accountable? Strange way to think of it champion, I'm just looking out for people. When my family and families all over the world have respectfully done the right thing," Kyrgios said, adding with a flourish:
"@TheBorisBecker is a bigger doughnut than I thought ... can hit a volley, obviously not the sharpest tool in the shed though."