MELBOURNE: Defending champion Sofia Kenin exited the Australian Open in tears on Thursday as world number one Ashleigh Barty negotiated a sticky moment to reach the third round.

Coco Gauff, 16, lost to Elina Svitolina, Stefanos Tsitsipas went the distance with Thanasi Kokkinakis and Fabio Fognini also needed five sets against fellow Italian Salvatore Caruso -- a match that ended in a furious row.

With Rafael Nadal resuming his quest for the all-time Grand Slam titles record in the night match, Kenin and Barty topped the day-time bill -- but their matches had very different outcomes.

Self-imposed pressure has weighed heavily on Kenin, 22, the world number four who was defending a Grand Slam title for the first time.

And after an emotional first-up win over wildcard Maddison Inglis, Kenin met her match in the in-form Estonian Kaia Kanepi, who won 6-3, 6-2.

"I know I couldn't really handle the pressure," said an emotional and red-eyed Kenin, who wiped away tears during her press conference.

While the American became just the third defending champion since 1970 to fail to reach the third round, Barty lifted hopes of a first Australian winner since 1978.

Playing with her left thigh heavily strapped, the top seed twice failed to serve out the win before ousting her 387th-ranked compatriot Daria Gavrilova 6-1, 7-6 (9/7).

Barty, who is unbeaten in seven matches after sitting out nearly all of last year, blamed the lapse on ring-rust.

"I haven't played a lot of tennis over the last 12 months. Obviously going to have ebbs and flows not only in concentration but level of play as well," said the Aussie, dismissing any injury concerns. "I'm as fit as a fiddle," she said.

Gauff beat Venus Williams and the then-champion Naomi Osaka en route to the round of 16 last year, when she was just 15, but Svitolina had other ideas this time around.

The Ukrainian world number five lifted her game in the pivotal moments to win 6-4, 6-3 as she eyes a maiden Grand Slam title.

Elsewhere, world number 192 Mackenzie McDonald beat Borna Coric 6-4, 6-2, 4-6, 6-4 to hand the Croatian 22nd seed the biggest upset of his career.

McDonald, 25, became the lowest-ranked tour-level winner against Coric, a US Open quarter-finalist last year, and reaches the third round of a Grand Slam for only the second time.

Tsitsipas was not expected to be troubled by 267th-ranked Kokkinakis but the Aussie, roared on by his close friend Nick Kyrgios, saved a fourth-set match point before going down to the Greek 6-7 (5/7), 6-4, 6-1, 6-7 (5/7), 6-4.

Spain's Feliciano Lopez, 39, was also taken five sets in his record-extending 75th consecutive Grand Slam before beating Italy's Lorenzo Sonego 5-7, 3-6, 6-3, 7-5, 6-4.

But the most combustible match was between Fognini and Caruso, as the two Italians had to be separated by a tournament official during a heated argument after the 16th seed's four-hour, 4-6, 6-2, 2-6, 6-3, 7-6 (14/12) win.

Russian rising star Andrey Rublev maintained his unbeaten start to the year with a 6-4, 6-4, 7-6 (10/8) victory over Brazil's Thiago Monteiro.

Sixth seed Karolina Pliskova ousted 2019 semi-finalist Danielle Collins, and Swiss 12th seed Belinda Bencic overcame veteran Russian Svetlana Kutznetsova 7-5, 2-6, 6-4.

Later Thursday, 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 rounds 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.