Players were seen interacting, embracing each other, playing basketball and partying like they did in pre-Covid-19 days but Matic said they were not required to adhere to any social distancing norms as Serbia had eased restrictions.
Earlier this week, Kyrgios said world number seven Zverev was "selfish" after a video emerged of Becker's fellow German apparently partying despite saying he was self-isolating having been in close contact with several infected players.
It didn't take long for the Australian to respond as he said: "For goodness sake Boris, I'm not competing or trying to throw anyone under the bus".
Kyrgios criticised Becker for defending Zverev, who he says should apologise for breaking his promise.More news: Breaking! India's first coronavirus vaccine candidate approved for human trials; details inside
Zverev, 23, took part in Novak Djokovic's Adria Tour exhibition event in Serbia and Croatia where Djokovic, Grigor Dimitrov, Borna Coric and Viktor Troicki all tested positive for the novel coronavirus last week.
German legend Becker, a six-time Grand Slam victor, took umbrage with Kyrgios' comment over viral footage of Zverev partying despite pledging to "follow self-isolating guidelines" after playing in Novak Djokovic's coronavirus-hit Adria Tour.
He then followed up his initial tweet with another similar message, noting: 'We all live in the pandemic called #Covid_19! "How selfish can you all get?"
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.More news: Hundreds queue for COVID-19 mass testing in Beijing
Kyrgios, who had slammed the decision to hold the Adria Tour as "bone-headed", said he was unhappy with the behaviour of some of the biggest names in the sport. "It's bad 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 said on Twitter.
"Rats? For holding someone accountable?"
"So I wake up and I see more controversial things happening all over the world, but one that stuck out for me was seeing Zverev again man, again, again, how selfish can you be?"
Becker conceded that his compatriot had "broken the quarantine rules and should be ashamed of himself" before again calling Kyrgios a rat and bizarrely telling him: "You're a amusing guy". I'm just looking out for people.More news: Lakers finalizing deal with JR Smith
As Becker tried to shift the focus from the initial debate, Kyrgios was once again not impressed as he wrote that the whole argument had nothing to do with tennis and instead of defending his fellow countrymen Becker should have encouraged Zverev to man up and tender an apology for his actions.