Australian Open champion Madison Keys must silence good friend and podcast co-host Jessica Pegula to reach the quarterfinals on Monday, while fellow title-holder Jannik Sinner also meets a compatriot.
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Keys and Pegula launch day nine at Rod Laver Arena in Melbourne in a fourth-round meeting between two players who know each other very well.
They are US teammates and also host a tennis podcast together, even planning to record an episode on the eve of their showdown.
"I guess it's kind of funny. We're so used to playing and being around each other," said Pegula, the sixth seed who is yet to win a Grand Slam.
Ninth seed Keys has not dropped a set in her title defence but has not always been wholly convincing in her three matches so far.
The winner will play either another American, the fourth seed Amanda Anisimova, or China's unseeded Wang Xinyu.
Also in the women's draw in the fourth round, fifth seed Elena Rybakina plays 21st seed Elise Mertens and second seed Iga Swiatek will be heavy favourite against Maddison Inglis.
Australian qualifier Inglis, ranked 168, features on Rod Laver Arena in the biggest match of her life.
The 28-year-old was given free passage into round four when two-time Melbourne champion Naomi Osaka pulled out injured.
SINNER FACES DARDERI
Reigning champion Sinner, seeded two, will similarly be a strong favourite against fellow Italian Luciano Darderi.
Darderi, the 22nd seed, has never made it this far at a Grand Slam, while Sinner is chasing a third Australian Open crown in a row.
Only Novak Djokovic has done that in the Open era.
Melbourne Park king Djokovic was supposed to play the 20-year-old Jakub Mensik.
But the Czech player pulled out injured on Sunday, sending 10-time champion Djokovic into the last eight without hitting a ball.
The 38-year-old Serb will play the winner of Monday's match between American ninth seed Taylor Fritz and the fifth seed Lorenzo Musetti of Italy.
In the last match of the day on Rod Laver Arena, the United States' eighth seed Ben Shelton plays Norway's 12th seed Casper Ruud in the timeslot when Djokovic was supposed to be on court.