Nick Kyrgios has been one of the most vocal players about Jannik Sinner's doping case ever since that news went public. He hasn't really strayed from that as he continues to talk about it, especially after the most recent chapter of the saga, which involved the settlement.
Kyrgios seems to have some beef with Sinner, though it largely seems one-sided as Sinner has refused to engage in it. From the side of Kyrgios, it became very personal after the positive doping test, which the Australian has never failed to mention since it happened.
Kyrgios has taken his rhetoric quite far at times, promising to make any match they play a "riot," which some fans have called excessive. Sinner, in the meantime, took a deal accepting a 3-month suspension so that he wouldn't risk a longer one. Kyrgios had a field day after that news, continuing his criticisms of the Italian. He wrote on X:
"Obviously Sinner's team have done everything in their power to just go ahead and take a 3 month ban, no titles lost, no prize money lost. Guilty or not? Sad day for tennis. Fairness in tennis does not exist."
His criticism of the Italian has become such a meme online that fans are making fun of him, and it's not just the fans. Tennis legend Boris Becker reacted to one of those memes, which appears to show Kyrgios in old age still complaining about the Sinner doping saga.
Kyrgios, who is very active on X, will surely not miss this one and is likely to respond because they have a history already.
The feud between Nick Kyrgios and Boris Becker

Believe it or not, these two have feuded in the past, which in some ways makes sense because Becker does comment on tennis stuff regularly while Kyrgios never misses a beat either. It started when Kyrgios mentioned Becker in an interview where he spoke about the difference between modern tennis and that which came before.
He basically said that the players of old weren't as good as those today because the modern game is much faster, which technically is true. Comparing eras, though, is never easy because it's impossible to do. Becker responded on X, calling out Kyrgios for commenting on a sport he famously hates as well as pointing out that, unlike some former players, he has never won a Major in singles.
Kyrgios responded by calling out Becker's private life, alluding to him hiding assets which resulted in prison time, and pointing out some of his own famous wins.