Rafael Nadal's ageing legs outlasted German teenager Alexander Zverev in an epic five sets as Serena Williams sailed into the last 16 at the Australian Open on Saturday.

Nadal needed all his trademark grit against Zverev, 19, dubbed "the future of tennis", and his superior fitness told as he ground out a thrilling 4-6, 6-3, 6-7 (5/7), 6-3, 6-2 win.

Germany's Zverev looked in discomfort as he won a punishing 37-shot rally nearly four hours into their contest as Nadal, still full of running, pressed his advantage in the final set.

It means Nadal, 30, follows fellow veteran Roger Federer into the last 16 of a contest now missing defending champion Novak Djokovic after his stunning defeat to Denis Istomin.

"Everybody knows how good Alexander is," said Nadal, who will play France's Gael Monfils in the fourth round. "He's the future of our sport, and the present."

Williams, by contrast, had no such trouble as she brushed off fellow American Nicole Gibbs 6-1, 6-3 to maintain her charge towards a 23rd Grand Slam title. Williams has been in brilliant form and will take some stopping as she zeroes in on Steffi Graf's Open-era record.

The next player to take on Williams will be Barbora Strycova, who ousted France's Caroline Garcia.

"Obviously I'm here for one reason," said Williams, who again lifted her index finger in celebration at her victory, symbolising the number one.

“But at the end of the day, this is all bonus for me and I look forward to playing [Strycova]. I'm ready for her."

Elsewhere 117th-ranked Istomin built on his win over Djokovic by beating Pablo Carreno Busta 6-4, 4-6, 6-4, 4-6, 6-2 to reach the last 16 for the first time.