Stefanos Tsitsipas now has something in common with Serena Williams.

The rising star from Greece suffered a big setback Tuesday in the first round of the U.S. Open — then attacked the chair umpire for bias after being given a code violation for receiving coaching during the match.

The 21-year-old, who is considered one of the future stars of the sport and has been ranked as high as No. 5 in the world, is on a slide, admitting his “brain” is tired and questioning his motivation after losing in four sets, 6-4, 6-7, 7-6, 7-5, to Russia’s Andrey Rublev.

“I feel my mind is just — I don’t feel inspired,’’ said Tsitsipas, who also dealt with cramping during the loss. “I don’t feel like I’m chasing something.’’

Early in the match, Tsitsipas got a warning from the chair umpire.

“You’re all weirdos,’’ Tsitsipas appeared to say during his byplay. It’s not surprising he has become fast friends with the eccentric Nick Kyrgios.

Then the No. 8 seed stepped it up in his post-match press conference to a point where Williams and Kyrgios would be proud.

“The chair umpire was very incorrect in what he was telling me during the match,” Tsitsipas said. “I don’t know what this chair umpire has in specific against my team, but he’s been complaining and telling me that my team talks all of the time when I’m out on the court playing.

“I believe he’s not right, because I never hear anything of what my team says from the outside. I feel like some of them have preferences when they are on the court.”

Part of Tsitsipas’ team is his father, Apostolos, who serves as the primary coach.

“My father outside, who usually does the talking, he’s trying to pump me up by saying, ‘Come on,'” Tsitsipas said. “Raising my confidence by not coaching but by trying to boost me up. I believe the coach for my opponent does the same thing, which is normal.”

Tsitsipas lost in the first round at Wimbledon and in his first match of the last US Open tuneup in Cincinnati. He admitted he’s still stinging from his epic five-hour loss to Stan Wawrinka at the French Open.

“I remember achieving my career high of 5 in the world,’’ Tsitsipas said. “I was really excited when I saw that No. 5 and my name next to it. Then I realized that in order to stay up there, I have to be very, very consistent: semifinals, finals, winning titles. And I don’t know why that got in my brain a little bit. … I feel like I’m doing the same thing over and over again, and my brain can’t really take it anymore.’’

Simona Halep is moving on. Now, she can move on.

After suffering back-to-back first-round losses in the U.S. Open — including last year becoming the first women’s No. 1 seed in the Open Era to lose that early — the reigning Wimbledon champion defeated American Nicole Gibbs 6-3, 3-6, 6-2 on Tuesday in the opening round in Flushing.

Halep, a former French Open champ whose best U.S. Open finish is a semifinal run in 2015, now faces American Taylor Townsend in the second round.

“Am I allowed to dream big again for this tournament?” Halep said.

John Isner won his 11th straight first-round match in Queens, breezing past Guillermo Garcia-Lopez, 6-3, 6-4, 6-4, in under two hours.

The 14th-seeded, top-ranked American — and only ranked American left in the field, following 26th-seeded Taylor Fritz’s first-round loss — will face Jan-Lennard Struff in the second round.

— additional reporting by Howie Kussoy