Naomi Osaka pulled out of a warm-up tournament for the Australian Open Saturday, saying her "body got a shock" after playing her first matches for four months, opening the Melbourne Summer Set title door to Simona Halep.

Osaka, the reigning Australian Open champion, hit the court this week for the first time since her tearful early exit at the US Open, after which she took a long break to deal with personal matters.

In Melbourne, she played three matches in quick succession and they took a toll. She withdrew before the start of her semi-final on Rod Laver Arena against Russian Veronika Kudermetova.

"Unfortunately I have an abdominal injury which I need to rest and prepare for the #AusOpen," the Japanese superstar and top seed said in a statement released by the Australian Open on Twitter. 

Osaka, who has fallen to 13 in the rankings, added on her own Twitter feed that it had been a tough return and she didn't want to push too hard with the opening Grand Slam of the year starting on January 17.

"Sad to withdraw due to injury from my match today, my body got a shock from playing back to back intense matches after the break I took," she said. 

"Thank you for all the love this past week I'll try to rest up and I'll see you soon."

The four-time Grand Slam champion had beaten France's 61st-ranked veteran Alize Cornet 6-4, 3-6, 6-3 in her first match back on Tuesday. 

She then demolished Belgium's Maryna Zanevska 6-1, 6-1 before blasting past Germany's Andrea Petkovic 6-1, 7-5 to make the semi-finals.

Her withdrawal sent Kudermetova into the final with a walkover where she will meet Halep, who crushed Chinese teenager Zheng Qinwen 6-3, 6-2. 

The win ensured the Romanian, whose season was plagued by injury last year, reached a WTA singles final for the 13th straight season.

Positive 

"I'm really happy that I can play the final for the first tournament of the year. I'm feeling good, and I'm happy with the way I played," said the two-time Grand Slam winner and former world number one.

"Last year definitely was the worst year that I ever had. The toughest mentally also.

"But I want to forget about that. I just want to get the feeling from now. I am positive. I am feeling fit on court. I'm moving well. I'm playing good."

Halep missed Roland-Garros, Wimbledon and the Olympics last year with a calf injury and ended her season early due to a knee issue, which culminated in her ranking slumping to 20.

There are two simultaneous WTA events being run as part of the Melbourne Summer Set in the lead-up to this month's Australian Open.

In the other tournament, American Amanda Anisimova swept past 26th-ranked Russian Daria Kasatkina to zero in on her second WTA title, racing through the semi-final 6-2, 6-0.

The 56-minute thrashing at Rod Laver Arena set up a final against Belarus' Aliaksandra Sasnovich, who beat American young gun Ann Li 7-6 (7/4), 2-6, 6-3.

Anisimova, 20, had lost her last 14 matches against top-30 players, but was too hot for third seed Kasatkina, converting seven of 11 break points while dominating on her serve.