Greek world number 8 Stefanos Tsitsipas has told a chair umpire "you're all weirdos" during a tirade against perceived bias in his US Open first-round defeat.

Key points: Greek number eight seed Tsitsipas lost in four sets to Russian Andrey Rublev

Greek number eight seed Tsitsipas lost in four sets to Russian Andrey Rublev Naomi Osaka avoided becoming the third reining champion to crash out of the US Open in the first round

Naomi Osaka avoided becoming the third reining champion to crash out of the US Open in the first round Australian pair Alexei Popyrin and Jordan Thompson both won through to the second round

Tsitsipas complained to Damien Dumusois that he was biased "because you're French probably and you're all weirdos".

The argument came midway through the fourth set of Tsitsipas's 6-4, 6-7 (5-7), 7-6 (9-7), 7-5 loss to Russian Andrey Rublev in the first round, when the number eight seed appeared to be battling cramps and was slow to return to the court after losing his serve.

Dumusois told Tsitsipas it was time to play, but Tsitsipas was still reaching into his bag for a new headband and screamed at Dumusois that he still needed time to change.

Dumusois told Tsitsipas he would be penalised.

"I don't care," Tsitsipas replied. "Do whatever you want, because you're the worst.

"I don't know what you have against me.

"Because you're French probably and you're all weirdos. You're all weirdos."

Dumusois is indeed French.

Tsitsipas had been angry that Dumusois believed he was getting coaching during the match from his father, Apostolos, which is not allowed.

"The chair umpire was very incorrect in what he was telling me during the match," Tsitsipas said afterwards.

"I don't know what this chair umpire has in specific against my team, but he's been complaining and telling me that my team talks all of the time when I'm out on the court playing. He's very … I don't know. I believe he's not right, because I never hear anything of what my team says from the outside."

Tsitsipas added that he thought tennis needed more neutrality in its officiating.

"I feel like some of them have preferences when they are on the court," he said.

Tsitsipas lost in the first round for the second straight major tournament after opening his season by beating Roger Federer en route to the Australian Open quarter-finals.

He said Dumusois's warnings had affected his performance.

"Well, it's not very pleasant when you have the umpire give you warnings and time violations and coaching violations during a match," he said.

"It can affect your thinking. It can affect your decision-making."

Tsitsipas was not the only seed to depart in the first round of the men's draw, with the fourth-seeded Dominic Thiem losing to Italy's Thomas Fabbiano 6-4, 3-6, 6-3, 6-2.

Naomi Osaka's title defence off to rocky start

Naomi Osaka was made to work for her victory. ( AP: Michael Owens )

Naomi Osaka's US Open title defence did not get off to the best of starts after the number one seed needed three sets to progress past Russian Anna Blinkova.

Back in Arthur Ashe Stadium where she beat Serena Williams in last year's chaotic final, Osaka trailed 3-0 at the outset against 84th-ranked Blinkova, couldn't close things out in the second set, then eventually came through for a 6-4, 6-7 (5-7), 6-2 victory in the first round.

Osaka wore a black sleeve over her left knee — the 21-year-old pulled out of a warm-up tournament earlier this month after suffering pain in the knee — but it wasn't so much her movement as her erratic strokes that presented problems for the Japanese world number one, who finished with 50 unforced errors, more than double that of Blinkova.

Osaka wasted a match point with a bad forehand while trying to serve it out at 6-5 in the second set, before gathering herself in the third.

Only two US Open women's champions have lost in the first round the following year during the professional era. It last happened in 2017, when Angelique Kerber was beaten by none other than Osaka.

Osaka, who won this year's Australian Open before suffering first-week defeats at the French Open and Wimbledon, regained the top ranking from Australian Ash Barty earlier this month but must win at Flushing Meadows to hold top spot.

Coco Gauff is fast becoming the "comeback kid" after she emerged from a set and break down to make a winning US Open debut at age 15.

The American beat Anastasia Potapova 3-6, 6-2, 6-4, with the players' combined age of 33 making it the youngest match-up of the first round at Flushing Meadows.

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With her parents jumping out of their front-row seats and a raucous partisan crowd backing her at Louis Armstrong Stadium, Gauff suffered another break in the deciding set before advancing.

"I was trying to calm myself down," Gauff said in her post-match interview.

"I was almost out."

Among the Australians in the women's draw, Ajla Tomljanovic defeated Marie Bouzkova 1-6, 7-5, 6-1 but Astra Sharma lost to Magda Linette, who triumphed 6-3, 6-4.

Millman goes down to Nadal

Rafael Nadal cut short John Millman's return to Arthur Ashe Stadium with a crushing first-round win over Australia's 2018 quarter-finalist.

Opening his quest for a fourth title at Flushing Meadows, second-seeded Nadal dominated Millman 6-3, 6-2, 6-2 in little more than two hours.

Millman caused the upset of the tournament last year when he beat Roger Federer in the fourth round before losing to Novak Djokovic in the last eight.

John Millman prepares to make a return in his loss to Rafael Nadal. ( AP: Adam Hunger )

"He showed last year what he is able to do when he is playing well and it was a match I came on court with a lot of respect," Nadal said of Millman.

Nadal will play another Australian in the second round after Millman's countryman Thanasi Kokkinakis vindicated his wildcard entry with a spirited four-set win over Belarusian qualifier Ilya Ivashka.

The injury-plagued Kokkinakis overcame the disappointment of failing to serve out the first-round match in the third set to tough out a 6-3, 7-6 (10-8), 6-7 (4-7), 6-2 victory.

Meanwhile, Australian young gun Alexei Popyrin surged into the second round of the US Open for the first time.

Popyrin, who only turned 20 this month, downed the vastly more experienced Federico Delbonis 6-1, 7-5, 7-6 (7-5) to notch yet another milestone in a breakout season for the former French Open junior champion.

After cracking the world's top 100 for the first time, then earning direct entry into a major for the first time, Popyrin has now reached at least the second round at all four majors in 2019.

He did not have it easy, though.

Alexei Popyrin reached a career-best third round at the 2019 Australian Open. ( Reuters: Lucy Nicholson, file )

After breezing through the opening set in 23 minutes, Popyrin had to battle back from service breaks down in the second and third to keep the Argentine at bay.

Sporting matching "Pop on Top" T-shirts, Popyrin's parents rode every point before their son finished on top in a tick over two hours.

His reward is a meeting with Mikhail Kukushkin after the Kazakh removed 10th-seeded Wimbledon semi-finalist Roberto Bautista-Agut from Popyrin's passage to the last 32 in New York.

Should he cash in on his good fortune, Popyrin could face countryman Jordan Thompson for a spot in the second week.

Thompson's 6-3, 6-2, 6-4 victory over Joao Sousa booked Popyrin's fellow Sydneysider a second-round date with Italy's 24th seed Matteo Berrettini, who ousted Richard Gasquet 6-4, 6-3, 2-6, 6-2.

AP/AAP