Serbia's Novak Djokovic waves at the crowd after his defeat against Australia's Alexei Popyrin during their men's singles third round match on day five of the US Open tennis tournament at the USTA Billie Jean King National Tennis Center in New York City, on August 30, 2024. Angela Weiss, AFP/File

MELBOURNE, Australia -- Novak Djokovic was at the center of another Australian Open row on Monday while Jannik Sinner needed medical attention in fierce heat to battle into the quarter-finals and Iga Swiatek won easily.

But American teenage qualifier Learner Tien's fairytale run is over after he suffered a thigh problem in a four-set defeat to Italy's Lorenzo Sonego in Melbourne.

Five-time major champion Swiatek torpedoed another feel-good story as she thrashed "lucky loser" Eva Lys 6-0, 6-1 to set up a meeting with eighth seed Emma Navarro.

Poland's Swiatek, who accepted a one-month doping ban late last year, then learnt that the World Anti-Doping Agency (WADA) will not appeal in the case. She called it "closure".

The 23-year-old has denied knowingly doping, saying non-prescription medication was contaminated.

With temperatures rising above 30 Celsius (86 Fahrenheit) for a second successive day, Ukraine's Elina Svitolina beat Veronika Kudermetova -- then declined to shake the Russian's hand.

But Svitolina's husband, the 38-year-old Frenchman Gael Monfils, retired from his match in the last 16.

READ: Sabalenka, Gauff on Melbourne collision course after reaching quarters

READ: Swiatek destroys Raducanu, Monfils stuns Fritz at Australian Open

- 'Insulting' -

The 24-time Grand Slam champion Djokovic wasn't in action -- he plays Carlos Alcaraz in a quarter-final blockbuster on Tuesday -- but he was the story.

On Sunday he refused to do a routine on-court interview, his hasty exit drawing boos from the Rod Laver Arena crowd.

Afterwards he said it was in protest at Tony Jones, a top sports presenter with host broadcaster Channel Nine, who he accused of "insulting and offensive comments" towards him and Serbian fans.

Jones on Friday labelled Djokovic a "has-been", among other things, and said: "Kick him out."

Djokovic said he would boycott interviews with the TV station until he got an apology.

He later released a short video on X to explain his stance and it has been viewed more than 60 million times, and caught the attention of the site's owner Elon Musk.

"It's way better just to talk to the public directly than go through the negativity filter of legacy media," Musk posted in response, with Djokovic replying: "Indeed."

Tennis stars past and present and Australian Prime Minister Anthony Albanese all waded into the row.

Jones apologized on air Monday and hours later organizers Tennis Australia said: "Novak acknowledges the apology has been given in public as requested, and is now moving on and focusing on his next match."

Djokovic has been at the center of controversy before at the Australian Open, notably in 2022 when he was deported on the eve of the tournament over his refusal to be vaccinated against Covid.

He claimed ahead of this year's tournament that he was "poisoned" by lead and mercury in his food while held in a hotel before being deported.

- 'Light for Ukraine' -Italy's Jannik Sinner plays a return to Australia's Christopher O'Connell during their men's singles third round match on day six of the US Open tennis tournament at the USTA Billie Jean King National Tennis Center in New York City, on August 31, 2024. Kena Betancur, AFP/File

Back on the tennis court, world number one and defending champion Sinner needed medical attention and overcame a long interruption when he broke the net with a thunderous serve to reach the last eight.

He came home 6-3, 3-6, 6-3, 6-2 against Danish 13th seed Holger Rune on Rod Laver Arena.

Sinner struggled in the hot conditions, his hand visibly shaking and his heart rate taken in the third set before he left court for a medical timeout.

Sinner said afterwards he had suffered with dizzy spells.

He next faces home hope Alex de Minaur, the eighth seed who beat unseeded Alex Michelsen of the US in straight sets.

Svitolina, the 28th seed, defeated Russia's unseeded Kudermetova 6-4, 6-1.

Svitolina refused to shake her opponent's hand afterwards, as is the case for all Ukrainian players facing Russians because of the war.

"For me to find a way to win matches, to find a way to bring a little light, a little win for the Ukrainian people is something that I feel I am responsible for," said the 30-year-old from Odesa.

Svitolina faces American 19th seed Madison Keys after she beat former finalist Elena Rybakina in three sets.

An exhausted Monfils was unable to make it a double celebration however when he called it quits with American 21st seed Ben Shelton leading two sets to one and a game up in the fourth.

Shelton faces Sonego after the Italian beat the 19-year-old Tien 6-3, 6-2, 3-6, 6-1.

Tien had shocked Daniil Medvedev in a late-night five-set thriller in the second round to announce himself on the world stage.

© Agence France-Presse