Third seed Dominic Thiem cited "physical issues" for his surprise fourth-round exit at the Australian Open.

The US Open champion lost in straight sets to Grigor Dimitrov overnight, 6-4, 6-4, 6-0.

Without revealing the extent, or nature, of what the physical issues were, Thiem said he didn't "want to find any excuses" for his poor performance.

"But the thing also is that I'm also not a machine. I mean, sometimes I would like to be, but there are really, really bad days," last year's finalist commented.

"I think a combination of some things, some little physical issues, plus a real bad day, plus the fact that he's a great player. So a combination of those three things, and a result like that can happen."

It will make for a novel quarter-final pairing with 18th seed Dimitrov to face qualifier Aslan Karatsev in the next round.

The 27-year-old Russian got the better of Felix Auger-Aliassime.

Novak Djokovic is in fourth round action this morning (Sunday) against Milos Raonic.

In the women's draw, Serena Williams and second seed Simona Halep are set to clash in a mouth-watering quarter-final.

Williams accounted for the challenge of Arnya Sabelenka 6-4, 2-6, 6-4.

The seven-time winner had established a 4-1 lead in the final set, but 22-year old Sabalenka levelled at 4-4 before Williams finished strongly.

Halep, meanwhile, bounced back from losing the first set to edge past the challenge of French Open champion Iga Swiatek.

She lost the first 3-6, before winning the final two sets 6-1, 6-4.

Another tournament favourite, Naomi Osaka, is also into the last eight.

She booked her place in dramatic fashion, surviving two match points against Garbine Muguruza.

She faced two match points in the second, having lost the opening set 4-6.

The Japanese player survived, however, and clinched a tight final set 7-5 to set up a clash with surprise quarter-finalist Su-Wei Hsieh