With his first match in more than four months behind him, defending US Open champion Carlos Alcaraz will look to step up his game against Portugal's Jaime Faria on Wednesday.
Alcaraz and women's defending champion Aryna Sabalenka will headline the night session on Arthur Ashe Stadium as second-round action gets underway.
Former champions Daniil Medvedev and Sloane Stephens and top US hopes Jessica Pegula, Ben Shelton and Frances Tiafoe are also in action.
AFP Sport looks at three matches to watch on Day 4 of the US Open:
CARLOS ALCARAZ (ESP X2) V JAIME FARIA (POR)
First meeting
– Defending champion Alcaraz dispelled doubts – including his own – with a 6-4, 6-4, 6-4 victory over Roman Safiullin in his first match since a wrist injury sidelined him in April.
Alcaraz reported that he played "pain-free" and after feeling "really, really nervous" in the run-up to his opener the Spaniard got back in the groove and can now concentrate on the details of his game.
So far his serve hasn't hit the speeds of pre-injury Alcaraz. He said that was partly a tactical decision against Safiullin but said his serve, like the rest of his game, has room for improvement.
"I'm gonna try to serve better," he said. "The point is going to be try to serve normal, but it's a shot that I just always struggle, I guess.
"But it will be back, for sure."
Whatever gaps remain in his game, Alcaraz will be heavily favoured against 72nd-ranked Faria, whose five-set victory over 74th-ranked Jenson Brooksby made him the third Portuguese player to win a match at all four majors.
ARYNA SABALENKA (BLR X1) V POLINA IATCENKO (RUS)
First meeting
– World No 1 Sabalenka is searching for the consistency that saw her win back-to-back titles at Indian Wells and Miami in March, and she'll need it if she's to become just the third woman to win three straight US Open titles.
Sabalenka, who got past Camila Osorio in straight sets in the first round, will be up against a battler in 22-year-old Polina Iatcenko, the 160th-ranked qualifier who fought from a set down to beat Mexico's Renata Zarazua.
REI SAKAMOTO (JPN) V FRANCES TIAFOE (USA X11)
– First meeting
Tiafoe could have his hands full with 20-year-old Rei Sakamoto, who came through qualifying with a victory over his onetime idol Kei Nishikori and belted an astonishing 38 aces in a five-set first-round victory over Aleksandar Vukic.
Sakamoto was the Australian Open boys' champion two years ago and will be keen to make his presence felt among the senior ranks.
Tiafoe, a two-time US Open semifinalist, is part of a raft of American men trying to end their 23-year Grand Slam title drought.
Tiafoe won a title in Halle this year and reached the final of the Cincinnati Masters this month. He has already survived a big-serving barrage this week, withstanding 28 aces from Martin Damm in a five-set win over his compatriot.