Champion Kenin out, top-ranked Barty through at Australian Open

Sofia Kenin's emotional Australian Open defence ended in a second-round defeat 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, 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 teammate.

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 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.