Caroline Wozniacki and Ana Ivanovic have become the latest high-profile players to sign up for the Auckland Classic in January.
The tournament has attracted a number of the top players due to the fact that it serves as a warm-up for the first grand slam of the year, the Australian Open.
With Serena and Venus Williams already having confirmed their participation, it means that four former world number ones will be competing at the event.
"This is uncharted territory for a sporting event in New Zealand," tournament director Karl Budge said. "Any one of these four players could headline the tournament on their own. They are the most marketable female athletes in the world."
Ivanovic won the tournament back in 2014 and is hoping to capture her second title next year.
"I have very fond memories of winning the title in Auckland 2014 and I am determined to make my mark again in 2017," she said.
Venus Williams won the tournament in 2015 as she beat Wozniacki in the final, while Serena has never featured in the competition.
Reigning champion Sloane Stephens will also be part of the 24-player field.