Jannik Sinner won his 50th match of the year on Thursday to stay on course for a US Open semi-final duel with Carlos Alcaraz while two-time champion Naomi Osaka's colourful return to New York ended in a dispiriting second round defeat.

World number one Sinner beat America's 49th-ranked Alex Michelsen 6-4, 6-0, 6-2 to make the last 32 for a fourth successive year.

The 23-year-old Italian unleashed 28 winners and broke serve eight times.

"He's a very tough opponent. We played each other in Cincinnati. I knew what to expect," said Australian Open champion Sinner.

Sinner went on to claim his fifth title of the season in Cincinnati.

A day later it was revealed that he had escaped a ban despite twice testing positive for an anabolic agent in March, authorities accepting his explanation that the result was the result of contamination.

Sinner will face Australia's Christopher O'Connell for a place in the last 16.

Alcaraz, the 2022 champion, is seeded to meet Sinner in the semi-finals and is bidding to become just the third man in the modern era to win the French Open, Wimbledon and US Open in the same year.

The charismatic 21-year-old Spaniard took on former quarter-finalist Botic van de Zandschulp in the late match on the Arthur Ashe Stadium.

'Cherry on top'

The winner of that tie will next face Jack Draper after the 25th-seeded Briton swept past Facundo Díaz Acosta of Argentina in straight sets.

Osaka, the 2018 and 2020 champion, went down to defeat to 2023 semi-finalist Karolina Muchova of the Czech Republic as her comeback from maternity leave remained stalled.

Four-time major winner Osaka missed last year's tournament after giving birth to daughter Shai but she made up for that absence in the first round.

The Japanese star brushed aside former French Open winner Jelena Ostapenko with a dazzling display to match her show-stopping outfit.

Osaka appeared on court wearing a giant green bow on the back of her jacket, a tutu and small bows tied to the heels of her shoes.

On Thursday night, she switched green for black but the 26-year-old went down to a 6-3, 7-6 (7/5) defeat, leaving her without a third round appearance at the majors since the 2022 Australian Open.

Osaka blew three set points in the second set before a wild forehand -- her 21st unforced error of the contest -- sealed her fate.

Muchova, a semi-finalist last year, will meet Russia's Anastasia Potapova for a last 16 spot.

"Honestly this year the biggest win for me is that I could play again," said Muchova who picked up a wrist injury at last year's US Open which sidelined her for nine months.

"I'm really grateful I'm able to play tennis and that I made it here. This is just the cherry on top to be here in this stadium. It's crazy."

World number one Iga Swiatek charged into the third round, routing 217th-ranked Ena Shibahara of Japan 6-0, 6-1 to record her 55th match win of the year.

Poland's Swiatek, the 2022 US Open winner who is seeking to add a second title in New York to her four French Open triumphs, overwhelmed the 26-year-old qualifier, who was in the main draw of a Grand Slam for the first time, in just 65 minutes.

She committed only six unforced errors and goes on to face 25th seed Anastasia Pavlyuchenkova who was a quarter-finalist in New York 13 years ago.

Rybakina withdraws

Swiatek then saw likely semi-final opponent Elena Rybakina withdraw from the tournament ahead of her second round clash with 143rd-ranked French qualifier Jessika Ponchet.

Rybakina issued a statement saying she was forced to withdraw "because of my injuries".

Karolina Pliskova, the 2016 runner-up, suffered a left ankle injury and was forced to retire from her match with fifth-seed Jasmine Paolini after just three points.

"I hope she recovers soon because it's sad to see her leave the court like this. We didn't really play a match," said French Open and Wimbledon runner-up Paolini.

Polish seventh seed Hubert Hurkacz became the highest ranked man to fall when he lost in straights sets to Australia's Jordan Thompson.

US 16th seed Sebastian Korda was also eliminated in straight sets at the hands of Tomas Machac of the Czech Republic.

However, fourth seed and 2021 champion Daniil Medvedev defeated Hungary's Fabian Marozsan in straight sets