Top-seeded Italian Jannik Sinner shrugged off the doping furor surrounding him and overcame a bad start to reach the second round of the U.S. Open with a 2-6, 6-2, 6-1, 6-2 victory over American Mackenzie McDonald on Tuesday.

Sinner became embroiled in controversy in the week leading up to the season's final Grand Slam, after the International Tennis Integrity Agency said he tested positive twice for an anabolic agent in March, and was slow out of the starting blocks.

The Australian Open champion committed several unforced errors in the opening set and McDonald took full advantage to stake himself to an early lead.

The American broke early in the second set to grab a 1-0 edge, but after spurning three break points in a lengthy game, Sinner leveled the set at 1-1.

The hard-fought break appeared to fill the world number one with confidence, and the tide of the contest turned as he raced to a 4-1 advantage, before holding serve and breaking once again to square the match at a set apiece.

Sinner never looked back from there and took five games in a row to close out the third set, before claiming the first three games of the fourth set without dropping a point.

McDonald won a couple of games late in the match to delay the inevitable, but Sinner made his cushion stand up and served out the set to claim an emphatic victory.

The 23-year-old will face another American, Alex Michelsen, in the second round.

Zhang Retires from First Round with Leg Injury

Zhang Zhizhen of China lost to the 25th seed, Jack Draper of Great Britain, after retiring due to an injury. After dropping the first set 6-3, Zhang seemed to be hindered by a knee issue and lost 10 straight games. He called a medical timeout, but treatment failed to fix the problem, and he retired from the match while trailing 4-0 in the third set.