Zverev seeking improvement after first-round scare, Gauff set for Badosa test

But the German will surely be hoping for an easier ride against 52nd-ranked Frenchman Quentin Halys in the second round on Thursday.

French Open winner Zverev again anchors the night session on Arthur Ashe Stadium, where two-time champion Naomi Osaka and US contenders Coco Gauff and Taylor Fritz are also in action.

French veteran Gael Monfils, fresh off a first-round victory on his 40th birthday, will try to keep his final Grand Slam going against American Lerner Tien and Australian Open champion Elena Rybakina - who can seize Aryna Sabalenka's world number one ranking with a run to the semi-finals - faces Spain's Jessica Bouzas.

AFP Sport looks at three matches to watch on Day 5 of the US Open:

Alexander Zverev vs Quentin Halys

Zverev leads head to head 1-0

Zverev labored for four hours and 53 minutes to subdue Sonego in a wild five-setter lasting until nearly two in the morning. He said there was "definitely room for improvement" but he didn't grudge the effort.

After capturing his long-awaited first Grand Slam at Roland Garros and reaching the final at Wimbledon, the German has struggled on the hard courts of North America.

The protracted tussle with Sonego, which ended in Zverev's seventh victory in seven matches against the Italian, was a valuable test, he said.

"Overall I think it was a great, tough battle, and I think it was a match that I needed after the last couple of weeks."

Halys, who won his first career title at Kitzbuehel in July, is also battle-tested. He beat Facundo Diaz Acosta in four sets for his first first-round victory at the US Open in five main draw appearances.

Paula Badosa vs Coco Gauff

Badosa leads head to head 5-3

Gauff, whose two Grand Slam titles include her maiden major at the US Open, looked in fine form in a brisk first-round victory, but the 23-year-old American has history with former world number two Badosa, now ranked 87th.

Badosa's 5-3 record against Gauff includes a victory in Berlin this year, part of a resurgent summer for the 28-year-old Spaniard.

Badosa will be operating on short rest after her first-round match against Daria Kasatkina was halted twice by rain on Tuesday and she had to return to finish it off on Wednesday.

She had two match points that she couldn't convert before play was suspended and let slip five more when the match resumed on Wednesday before sealing the win.

"We always say that tennis is mental, but this, I think, was another level of mental," Badosa said of the agonising night she spent after failing to get over the line on Tuesday.

Gael Monfils vs Learner Tien

Tien leads head to head 1-0

Gael Monfils, heading into retirement after this season, celebrated his 40th birthday with a first-round victory over Adolfo Daniel Vallejo on Tuesday in front of an adoring New York crowd.

Now the French showman will try to extend the final Grand Slam of his career against 20-year-old American Learner Tien, who won his second career title in Geneva in May.

Monfils will be out to avenge a loss to Tien last month at the Cincinnati Masters, where the American made it to the semi-finals.

The crowd could be torn on Louis Armstrong Stadium.

Tien may be among the crop of American men aiming to end their 23-year-old Grand Slam title drought, but Monfils is a fan favorite in Flushing Meadows - where he has been delighting crowds for most of Tien's lifetime.