World number one Ashleigh Barty overcame a jittery start to defeat in-form Elena Rybakina 6-3 6-2 and reach the Australian Open fourth round.

Home favourite Barty was broken in her first two service games, but broke back on both occasions before breaking again in the sixth game of the first set.

Kazakh Rybakina, the champion in Hobart last week, converted only two from 10 break points.

Barty will now face American 18th seed Alison Riske in the last 16.

Riske beat Barty's doubles partner Julia Gorges 1-6 7-6 (7-4) 6-2.

"It was a very tough one," the 23-year-old said. "I felt like I had to be switched on. Some of the games were long. It's probably the best I played this summer so far.

"It's nice to have another chance to come out and enjoy it. I'm loving every minute."

Riske beat Barty in the fourth round at Wimbledon in 2019 in what was Barty's first Grand Slam as world number one.

Last year's beaten finalist, Petra Kvitova, comfortably saw off Russian Ekaterina Alexandrova 6-1 6-2. The Czech will play Greek 22nd seed Maria Sakkari, who defeated American 10th seed Madison Keys 6-4 6-4 to reach the fourth round of a Grand Slam for the first time.