Former Australian Open finalist Simona Halep withstood a precarious fourth round test to fight through against Belgium's Elise Mertens 6-4 6-4, booking her place in the last eight.

The world number three joins Australian top seed Ash Barty as the only other top four-ranked player in the quarter-finals.

Halep, the reigning Wimbledon champion, rode mid-match momentum to snatch six straight games, clinching the first set and an early double-break in the second set.

Australian Open: Simona Halep storms into quarter-finals_1
Inset: Simona Halep stretches for a shot in her straight sets victory over Belgian 16th seed Elise Mertens. Photo: Getty

The relentless power from Mertens, who made the semifinals two years ago in her breakout season, proved problematic, as she steered the second set back with some aggressive shotmaking.

However, her waywardness proved costly, as she struck 36 winners and 38 unforced errors for the match, compared to Halep's decidedly more positive ratio of 21 and 8.

Sealing the match with a driving fist pump, Halep said she needed to rediscover her earlier composure in the latter stages of the match.

"I needed to stay focussed in the end. I got a little bit nervous at 4-3 but I was strong enough to win the match," Halep told Nine post-match.

"Actually I had to calm down myself because when I get a little bit nervous I get crazy on court. So I had to stay cool."

Mertens defeated Halep in their previous meeting in the final of the WTA-Premier level Doha Open, where she stormed back from a 6-3 2-0 deficit to hoist her most lucrative career title.

Halep will now face either Poland's Iga Swiatek or 28th seed Anett Kontaveit for a semi-finals berth.