Coco Gauff and Alexander Zverev are also playing, while sixth seed Alex de Minaur carries Australia's hopes in the night session.
Medvedev and Tien headline Sunday's games
When Learner Tien began knocking on the door of elite tennis in 2025, he crossed paths repeatedly with Daniil Medvedev and what began as an upset in the second round last year turned into an unexpected yet compelling trilogy.
The American arrived in Melbourne as the 25th seed, but last year he was outside the top 100 when he upset Medvedev in an epic five-setter that lasted nearly five hours.
Their paths crossed again in Beijing, where Medvedev was forced to retire in the decider, but the Russian finally beat him in Shanghai in another match that went the distance.
"The thing is that I kind of don't like to play him, but he must hate to play me as well. All our matches... it's long, brutal rallies," Medvedev said.
"There's going to be a lot of shot-making, a lot of good defence from both of us, some passing shots. I'm going to try to enjoy the game of tennis. Of course, try to do my best to maybe surprise him somewhere."
Tien said all three matches against the former world number one have been tough, as neither player makes it easy on their opponents.
"We both don't give up too many free points. I think naturally that makes the rallies very long, games very long," Tien said.
"We both have to work very hard for points and games. I think matches just kind of drag out."
Mboko in for new experience against Sabalenka
Victoria Mboko will tick off two major career firsts when she faces world number one Sabalenka in her maiden Grand Slam fourth round on Rod Laver Arena, and the 19-year-old could not be more thrilled about it.
The Canadian teenager faces the daunting task of toppling top seed Sabalenka to secure a quarter-final berth, but Mboko is embracing what she called a "super cool" encounter.
"I've never played a current number one in the world. That's going to be a very different experience," she said.
"I've never played on a Grand Slam centre court either. A lot of firsts. I'm just really excited. It's something not many people get to experience."
Mboko arrived in Melbourne as the 17th seed following a breakthrough 2025 that included capturing a WTA 1000 title, marking her rapid rise through the tennis ranks.
Sabalenka, a self-described late bloomer who did not peak until her mid-20s, has been impressed by how quickly today's young players are developing compared to her own journey.
"She's a great player. She's a fighter, she's playing really good, aggressive tennis," the Belarusian said.
"Maybe for me, it would be really tough to handle the success at a young age. But seeing these (young) girls ... achieving so much, playing such great tennis, being really mature, it's incredible."