Sofia Kenin's emotional Australian Open defence ended in a second-round defeat on Thursday but top seed Ashleigh Barty overcame a blip to reach the last 16.
The highly-strung Kenin has struggled to handle expectations in her maiden Grand Slam title defence and admitted to having a few tears during a tense opening-round win on Tuesday.
The fourth seed shed more on Margaret Court Arena Thursday after her 6-3, 6-2 defeat in 64 minutes to the Estonian world number 65 Kaia Kanepi, becoming the highest-ranked player to go out of the tournament.
"I felt like I wasn't there, my head wasn't there," a teary-eyed Kenin admitted later.
Meanwhile world number one Barty cast friendship aside with a straight-sets victory over compatriot Daria Gavrilova, but appeared to lose focus serving for the match before closing out for a 6-1, 7-6 (9/7) win against her Fed Cup team-mate.
Barty's left thigh was heavily strapped but she seemed to move with ease in the second-round encounter against 387th-ranked Gavrilova.
"This isn't very subtle," Barty said referring to the bandage. "I'm as fit as a fiddle.
"When you play another Aussie, rankings go out of the window, so it was always going to be a tricky match."
Sixth seed Karolina Pliskova of the Czech Republic had few alarms in beating American Danielle Collins, a semi-finalist at Melbourne Park two years ago, 7-5, 6-2.
Swiss 12th seed Belinda Bencic was pushed harder by veteran Russian Svetlana Kutznetsova before coming through 7-5, 2-6, 6-4 in 2hr 39min.
In the men's draw, Russian rising star Andrey Rublev, the seventh seed, reached the last 16 with a straight-sets victory over Brazil's Thiago Monteiro 6-4, 6-4, 7-6 (10/8).
Later on Thursday, Rafael Nadal will test his sore back again as he chases a record 21st Grand Slam crown.
The ailing Spanish second seed had to modify his service action during a straight-sets cruise past Serbia's Laslo Djere in the first round.
He will round off the night session on Rod Laver Arena against American qualifier Michael Mmoh, having never lost a match at the Australian Open to a player ranked as low as the world number 177.
"My back is not perfect. Every day that I'm able to go through, probably there are more chances to be better," said Nadal, who also pulled out of the ATP Cup last week.
Before that match. teenage sensation Coco Gauff is looking for her latest big-name scalp in the prime-time evening slot on Rod Laver Arena.
The potential victim this time is Ukrainian fifth seed Elina Svitolina, who will be all too aware that Gauff knocked out Venus Williams and Naomi Osaka on her way to the fourth round a year ago aged 15.
Collated results from day four of the Australian Open at Melbourne Park on Thursday:
Andrey Rublev (RUS x7) bt Thiago Monteiro (BRA) 6-4, 6-4, 7-6 (10/8)
Feliciano Lopez (ESP) bt Lorenzo Sonego (ITA x31) 5-7, 3-6, 6-3, 7-5, 6-4
Casper Ruud (NOR x24) bt Tommy Paul (USA) 3-6, 6-2, 6-4, 7-5
Radu Albot (MDA) bt Christopher O'Connell (AUS) 6-2, 7-5, 7-6 (10/8)
Lloyd Harris (RSA) bt Alexei Popyrin (AUS) 6-2, 1-6, 6-3, 6-7 (5/7), 6-3
Mackenzie McDonald (USA) bt Borna Coric (CRO x22) 6-4, 6-2, 4-6, 6-4
Filip Krajinovic (SRB x28) bt Pablo Andujar (ESP) 6-2, 5-7, 6-1, 6-4
Daniil Medvedev (RUS x4) bt Roberto Carballes (ESP) 6-2, 7-5, 6-1
Stefanos Tsitsipas (GRE x5) bt Thanasi Kokkinakis (AUS) 6-7 (5/7), 6-4, 6-1, 6-7 (5/7), 6-4
Mikael Ymer (SWE) bt Carlos Alcaraz (ESP) 2-6, 6-4, 6-4, 7-6 (7/5)
Karen Khachanov (RUS x19) bt Ricardas Berankis (LTU) 6-2, 6-4, 6-4
Matteo Berrettini (ITA x9) bt Tomas Machac (CZE) 6-3, 6-2, 4-6, 6-3
Fabio Fognini (ITA x16) bt Salvatore Caruso (ITA) 4-6, 6-2, 2-6, 6-3, 7-6 (14/12)
Alex De Minaur (AUS x21) bt Pablo Cuevas (URU) 6-3, 6-3, 7-5
Cameron Norrie (GBR) bt Roman Safiullin (RUS) 3-6, 7-5, 6-3, 7-6 (7/3)
Rafael Nadal (ESP x2) bt Michael Mmoh (USA) 6-1, 6-4, 6-2
Ashleigh Barty (AUS x1) bt Daria Gavrilova (AUS) 6-1, 7-6 (9/7)
Ekaterina Alexandrova (RUS x29) bt Barbora Krejcikova (CZE) 6-3, 7-6 (7/4)
Anett Kontaveit (EST x21) bt Heather Watson (GBR) 6-7 (5/7), 6-4, 6-2
Shelby Rogers (USA) bt Olga Danilovic (SRB) 6-2, 6-3
Belinda Bencic (SUI x11) bt Svetlana Kuznetsova (RUS) 7-5, 2-6, 6-4
Elise Mertens (BEL x18) bt Lin Zhu (CHN) 7-6 (10/8), 6-1
Karolina Muchova (CZE x25) bt Mona Barthel (GER) 6-4, 6-1
Karolina Pliskova (CZE x6) bt Danielle Collins (USA) 7-5, 6-2
Kaia Kanepi (EST) bt Sofia Kenin (USA x4) 6-3, 6-2
Donna Vekic (CRO x28) bt Nadia Podoroska (ARG) 6-2, 6-2
Jennifer Brady (USA x22) bt Madison Brengle (USA) 6-1, 6-2
Kaja Juvan (SLO) bt Mayar Sherif (EGY) 3-6, 7-6 (7/2), 6-3
Jessica Pegula (USA) bt Samantha Stosur (AUS) 6-0, 6-1
Kristina Mladenovic (FRA) bt Nao Hibino (JPN) 7-5, 6-1
Yulia Putintseva (KAZ x26) bt Alison Van Uytvanck (BEL) 6-4, 1-6, 6-2
Elina Svitolina (UKR x5) bt Coco Gauff (USA) 6-4, 6-3