Spanish top seed Carlos Alcaraz continued the defence of his US Open men's title with a straight-set win over South Africa's Lloyd Harris.

Alcaraz, 20, was made to work hard in the third set before sealing a 6-3 6-1 7-6 (7-4) victory in New York.

Alcaraz, aiming to become the first man to win back-to-back US Open titles since Roger Federer in 2008, will face Britain's Dan Evans in the third round.

Third seed Daniil Medvedev won 6-2 6-2 6-7 (6-8) 6-2 against Chris O'Connell.

Russia's Medvedev, who won his sole major at Flushing Meadows in 2021, was cruising by two sets and a break up before he allowed the Australian back into the contest.

Medvedev, 27, lost his serve as 69th-ranked O'Connell levelled for 4-4 in the third set and could not convert two more break points at 5-5.

The former world number one had two match points in the tie-break, producing a double fault on the second, then another on set point, as O'Connell prolonged the match.

Medvedev regrouped to move 4-1 ahead in the fourth set, fighting off three break points for 5-2 and finally closing out victory at almost 01:10 local time.

Meanwhile, John Isner brought the curtain down on his professional career as the American veteran lost a five-set thriller to compatriot Michael Mmoh.

Isner, 38, lost 3-6 4-6 7-6 (7-3) 6-4 7-6 (10-7) as Mmoh fought back from match point down to win a tense fifth-set tie-break.

Elsewhere, Italian Matteo Berrettini is out after retiring when trailing 6-4 5-3 to France's Arthur Rinderknech.

The former semi-finalist badly twisted his ankle chasing down the ball as Rinderknech served for the set and, after a long period on the floor in clear pain, decided he could not continue.

Sixth seed Jannik Sinner eased through with a 6-4 6-2 6-4 win over another Italian, Lorenzo Sonego.

German 12th seed Alexander Zverev, the 2020 finalist, set up an eye-catching meeting with Andy Murray's victor Grigor Dimitrov by battling past Daniel Altmaier in four sets.

Isner bows out with records to his name

It was perhaps fitting that Isner, who won the longest match in professional tennis history against Nicolas Mahut in 2010, finished with another five-set epic.

Defeat by Mmoh lasted three hours 57 minutes - seven hours and eight minutes less than his win at Wimbledon - as he was beaten despite taking a two-set lead.

The 6ft 10in big-server, who turned professional in 2007, reached eighth in the rankings and won 16 singles titles, served another 48 aces to extend his record - and retires with 14,470 to his name.

The Flushing Meadows crowd were on their feet at the end of the match, with Isner struggling to hold back the tears.

"It's tough," he said. "You know I like to think I work as hard as I can. This is why I've worked as hard as I have my whole life, to play in atmospheres like this.

"Of course I don't win them all, as we know. Just like today, to play in front of this crowd, to have the support I have, is pretty special. So thank you."

Later the home fans packed court 12, where Isner had one final match in the men's doubles with 30-year-old Jack Sock - who is also retiring after the tournament - against Albano Olivetti and Robert Galloway.

However, the American duo were beaten 6-2 3-6 7-6 (10-3) by their opponents and Isner particularly received more well-wishes as they left the court.

Sock, 30, departs the sport having won four ATP singles titles and 17 tour-level doubles trophies.