Simona Halep vs.  Anett Kontaveit
How much can we glean from the history between these two? On the one hand, Halep is 2-0 against Kontaveit, and she won both of those matches in routine straight-setters. On the other hand, they haven’t faced each other since 2017. Since then, Kontaveit has risen as high as No. 14 in the world. More important, with Nigel Sears in her corner, she has looked sharp in Melbourne so far. She conceded just one game to Belinda Bencic, and survived a late rally by Iga Swiatek. Still, neither of those players are Simona Halep. If anything, the Romanian, with Darren Cahill doing the coaching again, has looked even sharper. She hasn’t dropped a set in four matches. Halep can struggle against big hitters, but does Kontaveit have the power to trouble her? I’m going to refer back to their two previous meetings and say no.
 Anastasia Pavlyuchenkova vs. Garbine Muguruza
One is a former Grand Slam champ trying to find her old form in a new year. The other is a player with great power and potential, who has never quite lived up to it. Their runs to the quarters have been two of the most pleasant surprises of the tournament. Unfortunately, one of them must come to an end. Their head-to-head record strongly suggests it will be Pavlyuchenkova. Muguruza is 4-1 against the Russian, and her only defeat came when she was forced to retire while down a set in Stuttgart two years ago. Despite that one-sided history, this is a tough match to call. On form, Muguruza is playing as well as anyone in the event; in her last two matches, she beat two Top 10 opponents, Elina Svitolina and Kiki Bertens, without any trouble at all. Pavlyuchenkova’s last two wins, over Karolina Pliskova and Angelique Kerber, have been closer, but she has shown grit in winning them. Still, when you’re predicting a Grand Slam quarterfinal, it’s tough to go against the player who has actually reached a Slam semi before.
 Alexander Zverev vs.  Stan Wawrinka
In the first of two generational ATP clashes on Wednesday, the 22-year-old German will take on the 34-year-old Swiss. If experience and pedigree are your thing, you’ll go with Wawrinka. He’s a three-time major title winner, and he won the first of them in Melbourne, in 2014. If matchups are your thing, you’ll go with Zverev. He’s 2-0 against Wawrinka. Ditto with fitness: Zverev has been cruising, while Wawrinka had to give everything he had to get past No. 4 seed Daniil Medvedev in a draining five-setter. On the other hand, you could also look at it the opposite way. In surviving Medvedev, Stan showed he’s still a Slam warrior, while Zverev has yet to be properly tested.
 Rafael Nadal vs.  Dominic Thiem
For anyone trying to pick a winner between these two, there’s precious little surface-specific evidence to go on. Only one of their 13 meetings has come on hard courts. It was a memorable one, of course, a sweat-soaked, five-hour spectacle at the US Open in 2018 that ended in a fifth-set tiebreaker at 2:00 in the morning. But what does that tell us? It tells us we can expect incredibly physical play and a surfeit of brilliant shotmaking, but it doesn’t say much about who is going to win. Is it Thiem’s time at last? While he split with one of his coaches, Thomas Muster, last week, he looked as if he had put any turmoil behind him in his straight-set win over Gael Monfils on Monday. And he has beaten Nadal on clay four times. But as he showed against Nick Kyrgios in the fourth round, Rafa is still Rafa, and he’s still finding ways to stay a step ahead of the next generation.