Coming off one of her biggest career wins just 24-hours earlier, it took the top-ranked Aussie just two sets (6-3 6-3) for to defeat Belgian Elise Mertens and march into a second consecutive Sydney semi-final.

"I knew it was going to be very tough and I was going to have to hit a lot of balls. I'm really happy with the way I was able to get in and out of games today," Barty said.

After some shaky moments earlier in the opening set, Barty was able to win three games in a row after dropping serve and broke Mertens in the process. 

Playing a free-swinging game with nothing to lose the world no.15 made Mertens work hard across the court for every point but after 33 minutes Barty had the advantage.

In the second set, it was a bit of a different story.

Martens was able to save two break points in her first game but was almost broken. It was the Australians serving which was difficult to face, with Barty barely conceding a point.

The Belgian was broken in the fifth game and couldn't recover.

The 2018 finalist now faces seventh seed Kiki Bertens on Friday.