BRISBANE: Defending champion Maria Sharapova and top-seeded Simona Halep withdrew from the Australian Open warmup tournament in Brisbane within an hour of each other on Tuesday, without playing a match.

Sharapova was scheduled to play fellow Russian Ekaterina Makarova on Tuesday but withdrew hours before the first-round match rather than risk further damage to a sore left forearm ahead of the first Grand Slam tournament of the season.

The five-time major winner said she'd hurt her forearm in practice a couple of days earlier and had to withdraw as a precaution. Her place in the draw was taken by Margarita Gasparyan.

Halep had a first-round bye and withdrew a day ahead of her scheduled opening match against Victoria Azarenka because of a problem with her left Achilles tendon.

"I wanted to play here. This injury is very difficult," Halep said. "I thought I was ready, but I can't do a full match."

Halep said she hoped to play at the Sydney International next week. Sharapova is heading directly to Melbourne to prepare for the Australian Open, which starts Jan. 18.

Sharapova won the Brisbane and Rome titles in 2015 and lost the final of the Australian Open to Serena Williams, who added the French Open and Wimbledon titles and was two match victories from a season Grand Slam when she lost to Roberta Vinci in the US Open semifinals.

Williams withdrew from her season-opening match against Ukraine at the Hopman Cup in Perth on Monday because of inflammation in her knee, but was hoping to be fit to play for the US team against Australia Gold on Tuesday. Ukraine improved to 2-0 in Group A by clinching a winning 2-0 lead against Czech Republic when Alexandr Dolgopolov and Elina Svitolina won their singles matches in straight sets.

Vinci, meanwhile, moved into the Brisbane International quarterfinals with a 6-1, 6-1 victory over 2014 Australian Open finalist Dominika Cibulkova. She'll meet either 2009 champion Azarenka or Ysaline Bonaventure in the last eight.

Among the men advancing in Brisbane were No. 6-seeded David Goffin, who beat Thomaz Bellucci 6-4, 6-4, and No. 8 Dominic Thiem, who held off local James Duckworth 3-6, 6-4, 6-3.

In a big upset result at the ASB Classic in Auckland, New Zealand, defending champion and top-seeded Venus Williams lost 6-7 (4), 6-3, 6-2 to Russian teenager Daria Kasatkina.

The 18-year-old Kasatkina, ranked 75 and the third-youngest player inside the top 100, made the most of her wildcard entry to oust the No. 7-ranked Williams, who made 73 unforced errors.

Former French Open champion Francesca Schiavone lost 7-6 (3), 4-6, 6-3 to Austrian qualifier Tamira Paszek, while third-seeded Caroline Wozniacki advanced with a 6-4, 6-4 win over Danka Kovnic.