Jannik Sinner and Carlos Alcaraz survived US Open first round wobbles on Tuesday to stay on course for a semi-final showdown as two-time champion Naomi Osaka made a tearful and colourful return to Flushing Meadows.

World number one Sinner fought back from a set and break down to defeat 140th-ranked Mackenzie McDonald in his first match since being embroiled in a doping test controversy.

Alcaraz, the third seed and 2022 champion in New York, overcame a second set blip to defeat 186th-ranked qualifier Li Tu of Australia 6-2, 4-6, 6-3, 6-1.

Italian Sinner battled to a 2-6, 6-2, 6-1, 6-2 win over American opponent McDonald on Arthur Ashe Stadium, racing away with 18 of the last 22 games.

The 23-year-old was returning to action just a week after it was revealed he had failed two drug tests in March.

However, he escaped a lengthy ban when officials decided he wasn't at fault for the presence of banned anabolic agent clostebol in his system.

"The response from fans, I feel it has been great, also throughout when the news came out, in the practice sessions, there was a lot of support," said Sinner.

"It's still a little bit not easy. You have to go through day by day."

Alcaraz, who won the first of his four Grand Slam titles in New York in 2022, opened the night session against Tu who hasn't won a match on the main tour this year.

- 'He surprised me' -

However, Tu thrilled the crowd as he went about his task of facing a player who is attempting to become only the third man in the modern era to win the French Open, Wimbledon and US Open in the same year.

After being outplayed in the opening set, Tu levelled the tie on a fifth set point in the second.

But Alcaraz crushed any hint of a shock result by running away with the clash to record his 15th successive Grand Slam match win.

"He surprised me a little bit. In the first set, he was nervous but after that he grew into it and played great tennis," said the Spanish star.

Naomi Osaka, who missed last year's tournament after giving birth to her daughter Shai, stunned 10th seed Jelena Ostapenko.

Former world number one Osaka, now ranked 88, came through 6-3, 6-2 for her first top 10 win in four years.

The Japanese star crunched nine aces in her 19 winners and committed just five unforced errors to her opponent's 21.

Her tennis was as striking as her outfit. Osaka arrived on court with a brightly-coloured, giant bow on the back of her jacket and a frilly skirt over her playing dress.

To complete the ensemble, the 26-year-old attached small bows to the heels of both shoes.

"Last year I was watching Coco (Gauff) play and I so badly wanted to step on these courts again," said a tearful Osaka, a four-time major winner.

- 'Means so much' -

"I didn't know if I could ... just to win this match and just to be in this atmosphere means so much to me."

World number one and 2022 champion Iga Swiatek defeated 104th-ranked Kamilla Rakhimova after having to save three set points in the second set.

French Open champion Swiatek, chasing a second New York title to add to her four triumphs in Paris, saw off the Russian lucky loser 6-4, 7-6 (8/6) to extend her season-leading record to 54 match wins.

However, it was an unconvincing display by the Pole who committed 41 unforced errors.

Fifth-seeded Daniil Medvedev, who shattered Novak Djokovic's dream of a calendar Grand Slam by claiming the 2021 US Open, went through with a four-set victory over Dujan Lajovic.

Greek 11th seed Stefanos Tsitsipas, who has never made the second week at the tournament, was stunned 7-6 (7/5), 4-6, 6-3, 7-5 by Australia's Thanasi Kokkinakis.

Stan Wawrinka, the 2016 champion, failed in his bid to become only the third man in last 50 years to win a main draw match at the tournament at the age of 39 or over.

Wawrinka slipped to a 6-4, 7-6 (7/5), 6-3 loss at the hands of Italian qualifier Mattia Bellucci.

Two former women's champions were knocked out.

Bianca Andreescu, the 2019 winner, lost in three sets to sixth-seeded Jasmine Paolini while 2021 champion Emma Raducanu fell to 2020 Australian Open winner Sofia Kenin.

Britain's Dan Evans defeated 23rd seed Karen Khachanov 6-7 (6/8), 7-6 (7/2), 7-6 (7/4), 4-6, 6-4 in the longest match in US Open history, a five-hour, 35-minute epic.