Australian Open Day 5: Svitolina and Mertens through to last 16

No 4 seed Svitolina beats Ukrainian compateriot Marta Kostyuk, while Mertens prevails as opponent Alize Cornet needs medical treatment as temperatures reach 40 degrees.

Fourth seed Elina Svitolina is through to the last 16 at the Australian Open. Lukas Coch / EPA

Elina Svitolina showed not an ounce of sentiment as she gave 15-year-old compatriot Marta Kostyuk a 59-minute lesson in grand slam tennis to reach the fourth round of the Australia Open for the first time on Friday.

Qualifier Kostyuk became the youngest player to reach the third round of a grand slam in over 20 years on Wednesday but her dream run was brought to a shuddering halt in a 6-2, 6-2 loss to her fellow Ukrainian on Rod Laver Arena.

The teenager made an excellent start by breaking Svitolina in the first game of the match but from then on it was pretty much all one-way traffic as the in-form fourth seed set up a meeting with Czech qualifier Denisa Allertova.

Svitolina, showing no desire to spend a minute longer in the heat than necessary and no sign of the injury issues she reported after her second round match, went through when Kostyuk served up her ninth double fault of the contest.

"It's very special to me, I always love coming to Australia but I never got beyond the third round," Svitolina said on court.

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"It was sad that I was playing a Ukrainian girl, it's always difficult playing someone from your own country, but I'm happy that I'm still alive in this tournament."

Meanwhile, Elise Mertens reached the fourth round of the Australian Open in straights after a dramatic match in which her opponent Alize Cornet needed a doctor.

Belgian world No 37 Mertens won 7-5, 6-4 in just over two hours of energy-sapping tennis on Hisense Arena as temperatures touched 40 degrees

"It's hot," said the in-form Mertens, who is coming off a successful title defence at Hobart last week and is now into the last 16 of a grand slam for the first time. "The toughest I have had."

It was her French opponent however, who wilted in the heat in distressing scenes with the score at 2-1 in the second set.

Cornet was struggling to breathe as her blood pressure and pulse were monitored by the doctor on court during a medical timeout.

Ice towels were applied and ice packs rubbed over her face, arms and legs in a frantic attempt to cool the player ranked 42.

Cornet sat with her eyes closed, gasping for breath before resuming, clearly still wobbly.

She somehow recovered enough to fight back from 5-2 down to 5-4, saving three match points in the final game which went to five deuces before her gutsy resistance was ended.

"Credit to her for keeping going," said Mertens.