Alexei Popyrin has kept his end of the bargain to book a third-round US Open showdown with defending champion Novak Djokovic.

Popyrin defied searing heat in New York to grind down Spaniard Pedro Martinez 6-2 6-4 6-0 as three of his Australian cohorts crashed back to earth on day three.

The men's 28th seed was made to work hard in the heatwave conditions, twice having to fight back from break points down in the second set.

Popyrin, though, showed his mettle to pull through in 1 hour, 48 minutes, his supreme final set sending a statement to his rivals.

The 25-year-old Montreal Masters champion has yet to drop a set en route to the last 32 and blasted another 13 aces on Wednesday (Thursday AEST).

"It was really hot. Probably after the Olympics this year, this is probably one of the hottest conditions I've played in," Popyrin said.

"At moments, I felt quite rough but I trusted my fitness. I trusted my team and the most important thing, especially in this weather, was that we got through in straight sets and we have the maximum time to recover now.

"That was my biggest focus throughout the match."

Chasing an unprecedented 25th grand slam singles crown, second-seeded Djokovic had to battle hard to overcome fellow countryman Laslo Djere, winning an error-strewn match 6-4, 6-4, 2-0 before Djere retired.

"It's not what we want, it's not what the crowd wants," Djokovic said after Djere walked off court with an abdominal injury.

"He's such a good player, it should have been his second set," Djokovic added, with the world 109 leading 4-2 in the second set.

"It was a big fight, over two hours for two sets."

Djokvic's sub-par performance in stifling conditions under the roof on Arthur Ashe court will give Popyrin confidence.

Popyrin has recent experience against the super Serb, having lost in four sets twice this year already at the Australian Open and Wimbledon.

"I had chances in both matches and it's just a matter of taking those chances and playing good on the big points. That's what he did," said Australia's former French Open junior champion.

Djokovic, for his part, said he would have to be better.

"I served awful … I have to serve much better if I want to go far in this tournament," Djokovic said.

"Alex Popyrin has been playing some great tennis over the last few weeks."

Earlier, Rinky Hijikata was dealt a grand slam reality check as Australia's tennis stars crashed back to earth at the US Open in New York.

Hijikata was no match for Grigor Dimitrov, suffering a 6-1 6-1 7-6 (7-4) thrashing at the hands of the Bulgarian ninth seed to bow out in the second round.

Former women's quarterfinalist Ajla Tomljanovic and 18-year-old qualifier Maya Joint were also eliminated on day three at Flushing Meadows.

With 10 players making the second round, Australia's bumper 20-strong contingent had made their brightest start to the Open in 45 years.

The men, in particular, have been on fire with eight Australians winning their opening matches for the first time since 1976 — almost half a century ago.

Now they're down to seven, with 28th seed Alexei Popyrin given the chance to save the day later against Spaniard Pedro Martínez.

Hijikata stormed to the fourth round last year as a wildcard but knew he'd have his work cut out against Dimitrov, who is playing his best tennis at age 33.

Hijikata, though, could not have imagined winning only two games in the first two sets.

The 23-year-old put up more of a fight in the third but the one-time world number three's class ultimately prevailed as Dimitrov won through in two hours and 10 minutes.

Tomljanovic lost her second-round match 6-3 6-2 to Belgian 33rd seed Elise Mertens, while Joint's breakout grand slam run came to a halt with a 6-4 6-0 loss to 2017 runner-up Madison Keys.

But with four wins at her maiden international slam, Joint is projected to leave New York with a career-high ranking of number 108 in the world, up some 27 spots and on track to gain direct entry to the Australian Open in January.

Elsewhere, Wimbledon champion Barbora Krejčíková suffered an early defeat at Flushing Meadows, falling in straight sets to Elena-Gabriela Ruse 6-4 7-5.

The number eight seed won her second grand slam singles title this summer, but she didn't play any matches after the Paris Olympics.

She acknowledged last week not knowing where her level of play was. Turns out, it wasn't good enough.

"Winning Wimbledon is amazing. It's a great, great, great result, I'm very proud about it and how I was able to handle everything there," said Krejčíková, who hadn't played a tournament on hard courts since February.

"I was playing quite well, definitely better than in the first match. I think my game was improving, but it just wasn't enough."

Ruse got much more work on the US Open's hard courts while playing her way into the main draw through the qualifying tournament and was better than Krejčíková on the points that mattered most.

She fought back from 5-3 down in the second set to win the final four games.

"Barbora, she's such a good player, she's won so many matches in the last two years and it's just a dream for me," said Ruse, a 26-year-old from Romania.

She advanced to the third round of a slam for the first time and will play No. 26 seed Paula Badosa, who eliminated American Taylor Townsend 6-3 7-5.

Badosa continued her strong summer by reaching the third round of the for the first time.

AAP