Plenty of other former winners are in action, too. World No. 2 Rafael Nadal meets Tomas Berdych, Angelique Kerber faces 17-year-old American Amanda Anisimova, while Maria Sharapova and local hope Ash Barty clash on Rod Laver Arena.
How to watch
ESPN3 & ESPN+: Starting at 7 p.m. ET
ESPN2: Starting at 9 p.m. ET
ESPN2: Starting at 3 a.m. ET
Full Day 7 schedule here
Federer faces first real test Down Under
When Roger Federer plays Stefanos Tsitsipas in the fourth round of the Australian Open on Sunday (ESPN2, 3 a.m. ET), he may just see someone who reminds him of his younger self, a pretender, perhaps, to his crown.
Growing up in Greece, the 20-year-old Tsitsipas idolized Federer and watched many of his Grand Slam finals, including his 2008 win over Rafael Nadal at Wimbledon. Back then, Tsitsipas was just 9 years old.
The two men met for the first time at the Hopman Cup earlier this month, when Federer won 7-6 (5), 7-6 (4), but this will be the first time they have faced off on an official stage.
"I saw a lot of his matches," said Tsitsipas, who reached the fourth round of a Grand Slam for only the second time in his career after a tough four-set win against Nikoloz Basilashvili of Georgia. "If I thought about it now, it's insane I'm in this position where I can actually play him. It's really emotional."
Tsitsipas does not lack confidence. He believes he can reach the top, even if Federer, now 37, still has the intimidation factor on his side.
"It's not easy to play these kind of players that you've been watching for so long and you finally get to play them," Tsitsipas said. "Mentally you have to be much stronger than any other match that you have faced that week. Having such a name like Federer on the other side, it's an extra, I would say, advantage for him, because he's done what he's done. Mentally, for players to beat him, they have to be ready and believe in themselves."
Together with Nadal and Novak Djokovic, Federer has been responsible for foiling the hopes of rising stars, keeping them in their place and never allowing them to believe they can compete for the sport's biggest titles, week in, week out.
"I think you have to work for respect," Federer said, after cruising past American Taylor Fritz to reach the last 16 without dropping a set for the 36th time at a major.
"It's not that you have to create an aura, but make them know that if they're behind in the score, they're probably not going to win, and if they're ahead, that you're probably going to mount a comeback."
Tsitsipas trains at the Patrick Mouratoglou Academy in Nice, where among others, he has trained with Serena Williams. Mouratoglou, who first spotted Tsitsipas on YouTube, believes he is ready for the latter stages of Grand Slams.
"He's surprising all the time," Mouratoglou told ESPN. "He's the guy who goes so fast. In 2017, he was playing the qualifications [in Melbourne]. In 2018, he was in main draw everywhere. He ended the year No. 15, winning an ATP 250 [in Stockholm], reaching a Masters final [in Toronto], beating a lot of top 10s.
"He's a problem-solver, you put him in the court, you look at the other guy and you think, 'Wow, he's better than him,' but [Tsitsipas] is going to find a way to beat him. He's an incredible fighter, I didn't see him give up once. Almost every week he's like that. I love that he feels he is going to make it. He is very focused and driven. His two biggest qualities are his drive and how competitive he is."
Federer said he was looking forward to the test. "I'm happy I played against him at the Hopman Cup," he said. "I think he played really well there. I actually did, too. I thought it was really high-quality tennis. This is obviously a different type of match, it being best of five, it being a fourth round of a Slam.
"I'm happy for him. He's playing so well, and I'm looking forward to the match-up with him. I think it's going to be a good one. I like how he mixes up his game and also comes to the net. So will I. I think we will see some athletic attacking tennis being played."
Federer has been here before, many times. Tsitsipas has not, but he's ready.
Sharapova goes up against home hero Barty
Neither Sharapova nor Barty are top-10 seeds at this year's Australian Open, but their match Sunday is arguably the most compelling of the day.
Sharapova, the 2008 champion in Melbourne, played supreme tennis in the third round to knock out last year's winner, Caroline Wozniacki, in three sets.
Barty has enjoyed an efficient start to her home Grand Slam, winning all three of her matches in straight sets in an average match time of just 1 hour, 8 minutes.
There's no doubt Sharapova will be the favorite, but Barty has already proved she can mix it with the sport's elite. Just last week, the 22-year-old Queenslander knocked out world No. 1 Simona Halep from the Sydney International.
Can Tiafoe's Aussie dream run continue?
For the first time in his short career, Frances Tiafoe is through to the fourth round of a Grand Slam.
The 20-year-old American, who has become famous for his LeBron James-inspired celebrations, has already knocked out fifth seed Kevin Anderson and Italian journeyman Andreas Seppi.
But things don't get any easier, with former world No. 3 Grigor Dimitrov waiting in the fourth round.
Dimitrov won the only clash between these two in Canada last year, prevailing 7-6 (1), 3-6, 7-6 (4) in a tight encounter.