Reigning Australian Open champion Naomi Osaka blasted her way past Germany's Andrea Petkovic to reach the semi-finals of the Melbourne Summer Set on Friday.

The Japanese star raced through the opening set in just 26 minutes, but was pushed harder in the second before beating Petkovic 6-1, 7-5 to set up a semi-final against third seeded Russian Veronika Kudermetova.

Kudermetova beat fellow Russian Anastasia Potapova 6-2, 4-6, 6-0 to book her place in the semi-finals.

Osaka was in imperious form in the first set, smashing winner after winner from both sides, racing to a 5-0 lead before Petkovic was at last able to hold serve.

When Osaka broke Petkovic in the opening game of the second set it looked like an early night for the four-time Grand Slam champion.

However, Petkovic lifted her game as Osaka's dropped slightly and the German fought back to skip away to a 4-2 lead.

But Osaka fought back and levelled the set, then broke at 5-5 before comfortably closing out the match.

Osaka said falling behind in the set to someone who was playing better than her was good preparation ahead of this month's Australian Open.

"This is the reason I came and played this tournament, to get myself in challenging situations," she said.

"I felt like I learned from her while I was playing the match.

"It was really cool the way she didn't give up at any point and I felt like I should do the same thing and see what happens."

- Halep in semis -

Earlier, two-time Grand Slam champion Simona Halep laboured into the last four to stay on track for a final showdown with Osaka.

The former world number one, whose season was blighted by injury last year, overcame Switzerland's Viktorija Golubic 6-2, 5-7, 6-4 on Rod Laver Arena, but it was a hard slog.

"It was really difficult I have to admit," said Halep, who missed Roland-Garros, Wimbledon and the Olympics last year with a calf injury and ended her season early due to a knee issue.

"I didn't believe I could win this match but I fought till the end and I'm very proud of this."  

She was in charge in the first set but her killer instinct deserted her in the second, broken when serving for 5-3 then again when serving for the match, racking up 23 unforced errors.

They traded breaks in the third set until Halep dug deep, saving four break points in the seventh game then breaking in the ninth to clock a tough win.

Now ranked 20, she will play Zheng Qin-wen in the last four after the 126th-ranked Chinese player upset Croatia's Ana Konjuh 7-6 (7/1), 7-6 (8/6).

There are two simultaneous WTA events being run as part of the Melbourne Summer Set.

At the other event, American Amanda Anisimova came through a tough test against Romania's Irina-Camelia Begu 2-6, 6-3, 6-3 to set up a last-four clash against 26th-ranked Russian Daria Kasatkina who beat Spain's Nuria Parrizas Diaz 7-5, 6-1.

American Ann Li beat Russian Kamilla Rakhimova 6-1, 7-5 and Aliaksandra Sasnovich of Belarus progressed when Swiss teenager Clara Tauson retired with Sasnovich leading 6-3, 2-0.