NEW YORK — World No. 1 Jannik Sinner, playing his first match since news of his positive tests for a banned substance emerged a week ago, rallied from a slow start against American Mackenzie McDonald in the first round of the US Open on Tuesday.

Playing in the second match of the afternoon at Arthur Ashe Stadium, Sinner struggled in the opening set before rebounding for a 2-6, 6-2, 6-1, 6-2 victory.

“I tried to stay there mentally and I tried to get into rhythm, just tried to keep going, so I’m very happy to be in the next round,” Sinner said on court after the match. “It’s the first time I have won in this court so I am very happy.”

While McDonald, ranked No. 140 and seeking the biggest win of his career, certainly had the support of the home crowd, Sinner was largely cheered throughout the match and given a warm reception at its conclusion. There were no audible boos or jeers from those in the stands in response to the news that Sinner had tested positive twice for trace amounts of the anabolic steroid clostebol in an eight-day span in March.

The Italian player later told reporters the fans have been “great” and said there has been a lot of support during his practice sessions, as well as during Tuesday’s match.

“It has been very positive,” Sinner said. “I was very glad how the support was, also playing against an American … it’s a little bit different. So I’m happy how I handled the situation, it was not easy. So I think a lot of positive things from today’s day.”

Sinner, the reigning Australian Open champion and winner of the Cincinnati Open earlier this month, improved to 29-2 on hard court this season, the most wins on the surface by any player on the men’s or women’s tour this season.

While McDonald went up an early break in the second set before Sinner responded, Sinner firmly took control in the third set, needing just 24 minutes.

The 29-year-old McDonald became increasingly frustrated, telling his team he was “dying” at one point late in the match and being encouraged by those in his box to improve his body language. But he never found a way back and Sinner ultimately cruised to victory.

Last week, the International Tennis Integrity Agency (ITIA) announced Sinner had tested positive for low levels of clostebol, a banned anabolic steroid that can be used for ophthalmological and dermatological use, on both March 10 and March 18. He was provisionally suspended at the time but was able to compete during the appeal process.

During a hearing this month, the ITIA said an independent panel “determined a finding of No Fault or Negligence applied in the case,” and blamed the contamination on Sinner’s physiotherapist Giacomo Naldi.

According to the ruling, Sinner’s fitness coach Umberto Ferrara had purchased an over-the-counter spray, which contained clostebol, and Naldi had used it on himself to treat a small wound before giving Sinner a massage. Sinner has since parted ways with Naldi and Ferrara, telling reporters he was “not feeling that confident to continue with them” in a news conference Friday.

Sinner, 23, admitted he had been “worried” while waiting for the panel to decide his fate and had called the timing “not ideal” ahead of the year’s final major.

On Tuesday, he told reporters the reaction from his peers in the locker room had been “not bad” over the past few days.

“I cannot really control what they think and what they [say],” Sinner said. “That’s how everything went and how it was, [and] I already talk, no? 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.”

Sinner, whose best result at the US Open was a quarterfinal appearance in 2022, will next play American Alex Michelsen, McDonald’s doubles partner, in the second round Thursday.

ESPN Stats & Information contributed to this report.