World number one Jannik Sinner outclassed Alex Michelsen in straight sets to reach the third round of the US Open.

Italy's Sinner beat the American 6-4 6-0 6-2 for his 50th victory of the year.

Australian Open champion Sinner is playing at his first tournament since it emerged he had been cleared of any wrongdoing after twice testing positive for a banned substance earlier this year.

The 23-year-old will play Australian wildcard Chris O'Connell in the third round in New York.

Sinner had wobbled against American Mackenzie McDonald in the first round on Tuesday, losing the first set before rallying to triumph 2-6 6-2 6-1 6-2 at Flushing Meadows.

Against Michelsen on Thursday he had no such trouble. After coming out on top of a first set with several trades of breaks, Sinner did not look back, wrapping up a dominant victory in one hour 39 minutes.

"[I'm] very happy," said Sinner, who beat world number 49 Michelsen in the second round of the Cincinnati Masters earlier this month.

"He's a very tough opponent. We played each other in Cincinnati a week ago. I knew what to expect, he knew what to expect a little."

Elsewhere, seventh seed Hubert Hurkacz was knocked out in a 7-6 (7-2) 6-1 7-5 defeat by Australian Jordan Thompson.

Another seed soon fell as American 16th seed Seb Korda was beaten 6-4 6-2 6-4 by Czech Tomas Machac.

Machac will play Belgium's David Goffin or France's Adrian Mannarino - who play later on Thursday - in the third round.