Former world No.1 tennis players Caroline Wozniacki and Ana Ivanovic have been added to an already strong field for next year’s WTA tournament in Auckland.

The pair round out a line-up for the January event which also includes the Williams sisters, Serena and Venus, both former world No.1-ranked players.

There are just 22 women who have held the title of world No.1 in the history of the WTA tour, with only six currently competing.

Tournament director Karl Budge said the four had opted for the Auckland Classic as a warm-up for the Australian Open ahead of rival tournaments in Brisbane and Shenzhen.

“This is unchartered territory for a sporting event in New Zealand. Any one of these four players could headline the tournament on their own,” he said.

Serena Williams is currently ranked No.2 in the world behind Germany’s Angelique Kerber while Venus, a five-times Wimbledon champion, is 17th.

The Williams sisters have jointly captured 120 WTA Tour titles, 29 Grand Slam singles and 18 Grand Slam doubles titles.

Wozniacki of Denmark sits at No.19 on the WTA rankings, and is completing a comeback after a foot injury forced her from the court in October.

She was beaten in the US Open semi-finals 6-4 6-3 by eventual winner Kerber.

Serbian Ivanovic, who won the ASB Classic in 2014, is ranked 64th in the world.

She has collected 15 WTA singles titles and played in every Grand Slam for the last 12 years, winning the French Open in 2008.

Another late addition to the Auckland draw is defending champion Sloane Stephens.

The 23-year-old American beat Wozniacki in this year’s ASB Classic semi-final before defeating Germany’s Julia Goerges – another confirmed starter for next January – 7-5 6-2.