Jannik Sinner and Carlos Alcaraz survived US Open first round wobbles on Tuesday to stay on course for a semi-final showdown.

Alcaraz roared home for a 6-2, 4-6, 6-3, 6-1 victory over 186th-ranked Australian Li Tu to launch his bid to add the US Open to the Roland Garros and Wimbledon titles he claimed this year.

The 21-year-old Spaniard pushed his Grand Slam winning streak to 15 matches as he tries to join Rod Laver and Rafael Nadal as the only men in the modern era to win the French Open, Wimbledon and the US Open in the same year.

He got off to a hot start, seizing a 4-0 lead on the way to pocketing the first set.

But Tu, who came through qualifying, got to grips with the imposing Arthur Ashe Stadium and Alcaraz's serve to take the second set.

He tried to keep the pressure on in the third, but Alcaraz's break for a 4-3 lead opened the floodgates, the Spaniard winning the eight straight games to take the third set and build a 5-0 lead in the fourth.

"Obviously it surprised me a little bit. In the first set, it was obvious that he started the match nervous," Alcaraz said of Tu.

"But after the first he started to enjoy, to play really great tennis. I was playing great – probably a few mistakes," added Alcaraz, who had 50 winners and 30 unforced errors.

The third-seeded Spaniard, who won the first of his four Grand Slam titles in New York in 2022, will face Botic van de Zandschulp in the second round, headed toward a possible semi-final showdown with top-ranked Australian Open champion Jannik Sinner.

World number one Sinner, meanwhile, fought back from a set and break down to defeat Mackenzie McDonald at the US Open on Tuesday in his first match since being embroiled in a doping test controversy.

Italian Sinner battled to a 2-6, 6-2, 6-1, 6-2 win over his 140th-ranked opponent on Arthur Ashe Stadium, racing away with 18 of the last 22 games.

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

"The first match is not easy. McDonald played really well in the beginning so I just tried to stay there mentally and find some rhythm to keep going," Sinner said. "I have room to improve and I'm looking forward to that."

Victory on Tuesday was also a first win for Sinner on the showpiece Arthur Ashe Stadium. (AFP)