Ukrainian fourth seed Elina Svitolina outclassed 15-year-old compatriot Marta Kostyuk to reach the Australian Open fourth round.

Svitolina, 23, won 6-2 6-2 in 59 minutes against world number 521 Kostyuk.

The youngest player since 1997 to reach the third round of a Grand Slam, Kostyuk was aiming to be the youngest since 1996 to progress to the last 16.

Svitolina will play world number 130 Denisa Allertova in round four.

“It was sad that I was playing a Ukrainian girl,” said Svitolina. “It’s always difficult playing someone from your own country, but I’m happy that I’m still alive in this tournament.

“She’s a great fighter, she fought right until the end and has a great future.”

The Ukrainian pair’s match was played in 40C heat on Rod Laver Arena while France’s Alize Cornet had her blood pressure taken by a doctor during her defeat by Elise Mertens on the Hisense Arena.

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Roger Federer has called for the Grand Slams to up their levels of prize money and said players are “bored’ of having to push the issue.

The subject has been raised at the Australian Open after Novak Djokovic reportedly initiated a players-only meeting on the eve of the tournament.

Federer, 36, is a former president of the ATP player council, the same position Djokovic now holds.

“They could definitely pay more, no doubt about it,” Federer said.

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In 2012 there was talk of a player boycott if the major tournaments did not take action to increase prize money.

That was followed by pay increases, particularly for players losing in the early rounds.

“We’re not partners. We’re just players. It’s always hard to rally,” said Federer, a 19-time Grand Slam winner.

“We had a good agreement, in my opinion, that made the Grand Slams happy, the players pretty happy. It seems like that has run its course.

“The moment that happens, there’s not the same increases any more, so players have to rally, get back together again, put in the effort. The Grand Slams know that. They will only react when we do so. We’re ready to do it. It’s going to be the same process over and over again.

“It’s a bit boring, to be honest, always having to ask for stuff. If you look at the revenue, the sharing process, it’s not quite where it’s supposed to be.

“But you can’t go from here to right there in a day. We know that. We just hope they realise and they do appreciate us maybe more all the time and not just in waves.

“We have good contacts with the slams. It’s all good. But it’s going to be a never-ending story.”

It has been reported that Djokovic raised the prospect of forming a players’ union separate from the ATP.

Roger Federer enjoyed the kinder conditions of the night session as he beat Germany’s Jan-Lennard Struff at the Australian Open.

After the temperature earlier neared 40C, second seed Federer saw off Struff 6-4 6-4 7-6 (7-4) in closer to 30C.

The five-time champion goes on to face French 29th seed Richard Gasquet in the third round.

Federer’s fellow Swiss Stan Wawrinka was beaten in his second match since returning from knee surgery.

The 2014 champion, seeded ninth, only decided to play in Melbourne on the eve of the tournament and was well below his best as he lost 6-2 6-1 6-4 to Tennys Sandgren of the USA.

“I think the last 12 days was more than what I could have dreamed coming here,” Wawrinka said.

“I really came without thinking I will be able to play the first match. That’s a big step for me.”

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There were no such problems on Rod Laver Arena for Federer despite an impressive performance from Struff, the world number 55.

Federer, 36, broke serve once in each of the first two sets but had to recover from a break down in the third before sealing victory in the tie-break.

Asked if he had requested an evening match, Federer said: “It’s not my call, it’s [the tournament officials’] call.

“I wouldn’t have minded playing during the day because if you want to get to the top, you’ve got to thrive in all conditions.”

Belgian seventh seed David Goffin was knocked out in the second round, going down 1-6 7-6 (7-5) 6-1 7-6 (7-4) to 36-year-old Julien Benneteau, the world number 59.

Six-time champion Novak Djokovic made it through in “brutal” mid-afternoon conditions with a 4-6 6-3 6-1 6-3 win over France’s Gael Monfils.

Alexander Zverev, the fourth seed, booked a third-round meeting with Hyeon Chung of South Korea after beating fellow German Peter Gojowczyk 6-1 6-3 4-6 6-3.

Fifth seed Dominic Thiem, 24, recovered from two sets down to beat American Denis Kudla and reach the last 32.

The Austrian lost a tie-break in the opener but fought back to win 6-7 (6-8) 3-6 6-2 6-2 6-3.

In round three Thiem will face France’s 26th seed Adrian Mannarino, who beat Czech Jiri Vesely 6-3 7-6 (7-4) 5-7 6-3.

Argentina’s Juan Martin del Potro, the 12th seed, took four sets to see off Russian Karen Khachanov 6-4 7-6 (7-4) 6-7 (0-7) 6-4.

American Sam Querrey, the 13th seed, had a surprise exit to world number 80 Marton Fucsovics, going down 6-4 7-6 (8-6) 4-6 6-2 to the Hungarian.

Czech Tomas Berdych, twice a semi-finalist in Melbourne, beat Spain’s Guillermo Garcia-Lopez 6-3 2-6 6-2 6-3.