Melbourne - Serena Williams faces her biggest tennis test since giving birth on Monday against Simona Halep in a blockbuster last-16 clash at the Australian Open, while Novak Djokovic is up against fast-rising Daniil Medvedev.
Williams, at 37, has looked in ominous form so far at Melbourne Park, dropping just nine games, as she seeks one more Grand Slam title to put her alongside Margaret Court's record 24.
But top seed Halep is a much tougher assignment as she edges towards an eighth Melbourne title at a tournament blown wide open with the ousting of defending champion Caroline Wozniacki and second seed Angelique Kerber.
Williams hasn't faced a player ranked one since returning from giving birth last year.
"I haven't played the world number one since I've been back, I don't think. So it will be good. I'll be ready," she said ahead of the night match on Rod Laver Arena, adding that she was not dwelling on Court's record.
"I've been edging closer for probably like a decade now. I'm not even dealing with that right now."
French Open champion Halep struggled in her opening two matches, where she was extended to three sets in both, before finally hitting form to beat Serena's sister Venus in round three.
"I have nothing to lose," she said of facing Serena.
"I'm playing against a great champion. It's going to a bigger challenge but I'm ready to face it."
The winner will play either seventh seed Karolina Pliskova, who won the Brisbane International warm-up, or two-time major winner and 18th seed Garbine Muguruza for a place in the semi-finals.
Fourth seed Naomi Osaka is also in action against 13th seed Anastasjia Sevastova while Ukraine's Elina Svitolina takes on American Madison Keys.
Six-time champion Djokovic was taken to four sets by teenage Canadian Denis Shapovalov in the third round and meets another young gun in Russia's Medvedev, seeded 15.
"I feel like I've been playing well, so things are progressing well," said the Serbian 14-time Grand Slam winner.
Whoever gets through that encounter will have a quarter-final against either Japan's eighth seed Kei Nishikori or Spain's Pablo Carreno Busta.
Meanwhile, fourth seed Alexander Zverev has his work cut out against big-serving Canadian Milos Raonic in their last-16 match-up.
"He's playing very well," the German said of Raonic, who reached the semi-finals in Melbourne in 2016. "It's going to be a fun match. Obviously not a lot of rallies, not a lot of long points."
Awaiting the winner will be Croat Borna Coric or Frenchman Lucas Pouille.