The world number one Italian, who turns 23 on Friday, has never been past the third round at the American Midwest venue but is hoping to improve on that showing this week.

Victory in just under two hours marked the 25th of the season for Sinner, who is the first man into the third round at the ATP-WTA tournament, the last big test before the August 26th US Open start.

The top seed has not lost before the quarter-final stage of any event since Masters 1000 Shanghai last October.

Teenaged Michelsen went down with 42 unforced errors, his winner count of 14 exactly half of Sinner's total.

"It was a tough match," Sinner said. "I needed to start feeling the conditions and getting used to them.

"I'm very happy to be into the next round. I've struggled here in past years. But today I stayed positive and that helped in the win."

Sinner, who missed the Paris Olympics with tonsillitis and went out last week in the Montreal quarter-finals, said he was not at his best.

"I'm not yet at 100%, but the main goal is the US Open," he said. "I have one week (after Cincinnati) to prepare in the best possible way.

"The matches I can play here will help at the Open."

Ninth seed Stefano Tsitsipas played for the first time since splitting last week with his father as coach, with the Greek turning the tables in dramatic fashion to reach the second round 4-6, 6-4, 6-3 over Jan-Lennard Struff.

Tsitsipas trailed the German by a set and 3-1 but suddenly came alive as his game started to fall into place.

The 2022 Cincinnati finalist finished his victory with a pair of aces.