Carlos Alcaraz wasn’t afraid to send a warning message to Jannik Sinner as the pair practiced together at Flushing Meadows. The two players are on course to meet at the semi-final stage of the US Open, with both in second round action later today.

Also on hand to help out was Alvaro Alcaraz, the brother of Carlos, who hit balls with his sibling while Sinner rested.

And footage on social media showed the world No.3 struggling to return serve, prompting him to aim a cheeky comment in Sinner’s direction.

“Watch out Jannik, this one serves better than you,” he said, pointing at Alvaro to emphasise his point. The Italian appeared to react to the taunt, but his reply wasn’t clear on the video posted on X.

Alvaro is no novice when it comes to the courts. He plays regularly in ITF events, most recently at the M25 Murcia tournament in Spain where he was beaten in the first round by compatriot Sergi Perez.

The humorous incident is likely to have been welcomed by world No.1 Sinner, who has begun his campaign in America amid a siege of criticism. It emerged this month the 23-year-old had escaped a suspension despite failing two doping tests at Indian Wells in March.

Sinner was cleared of fault or negligence by an independent tribunal, having blamed the presence of clostebol in his system on hand massages from physio Giacomo Naldi, who had used a spray containing the substance to treat a cut on his hand. Naldi has since been removed from Sinner’s team.

The Italian has admitted he’s received a frosty reception from some fellow professionals, saying: “There are some reactions from other players. I cannot really control what they think.

“You know, that’s how everything went and how it was. I cannot control the players’ reaction, and if I have something to say to someone, I go there privately, because I’m this kind of person.”

He was able to park the controversy in his first round win over American Michael Mackenzie. That has set up a clash with another home player, Alex Michelsen, later on today.

Alcaraz, meanwhile, also got through in four sets against unseeded Australian Li Tu. The 2022 US Open champion will now take on Botic van de Zandschulp under the lights on the Arthur Ashe Stadium.