As world No.3 Garbine Muguruza joined the exodus of stars from the Open's summer lead-up events, Kontaveit was the first player to advance to the Sydney Tennis Classic semi-finals.

The fourth seed progressed when Tunisian Ons Jabeur retired from their quarter-final on Thursday due to a back injury shortly after losing the opening set 6-4.

Jabeur attempted to start the second set but, after two serves, the seventh seed signalled her match was over.

It was Kontaveit's first tour-level win against Jabeur and continued her strong form after claiming four titles in 2021 and becoming the first Estonian to qualify for the year-end WTA Finals.

"I really hope that Ons is going to be fine for next week and that it's nothing too serious," the world No.7 said.

"The first set was really competitive and we've had some tough matches before, but she's always come out on top.

"I was really happy to get the set and that I was playing the way I was playing."

Marking herself as an Open dark horse, the 26-year-old Kontaveit will face French Open champion Barbora Krejcikova for a spot in the Sydney final after the Czech third seed routed Frenchwoman Caroline Garcia 6-0 6-2.

Spanish fifth seed Paula Badosa will play Daria Kasatkina in Friday's other semi-final after the Russian accounted for Muguruza 6-4 6-4 on Thursday night.

Triumphant at the WTA Finals in Mexico in November, after Barty opted against defending her title, Muguruza had shaped as the most likely to crash the world No.1's party at this month's Open.

Instead, like out-of-form defending Open champion Naomi Osaka, Muguruza will enter the season's first grand slam somewhat underdone.

The former Wimbledon and French Open champion -- and 2020 Melbourne Park runner-up -- had been talking up her chances this week.

Alas, Muguruza went down in straight sets to Kasatkina in their showdown on Ken Rosewall Arena.

Badosa later booked her semi-final spot with a 7-6 (8-6) 3-6 6-3 win over weary Olympic champion Belinda Bencic, who was playing her second match of the day.

Playing her first match since contracting COVID-19 last month, the Swiss earlier clawed her way to a 2-6 7-5 6-3 victory over French lucky loser Oceane Dodin.

That match was suspended late on Wednesday with Bencic leading 6-5 in the second set.

But backing up to beat the ninth-ranked Badosa proved a bridge too far for the one-time world No.4.

Australian Associated Press