Auckland's ASB women's tennis classic continues to set the bar for international sporting events in New Zealand.

While the tournament is a small stop on the WTA tour calendar, year after year it attracts world class fields.

This year's event had both the Williams sisters Serena and Venus and former world number one Caroline Wozniacki and next year's lineup is already shaping up as one of the best.

Confirmation that Wozniacki will return as top seed is a real coup for an event that regularly wins best International series tournament on tour.

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The Dane is ranked sixth in the world and has played in six finals this year, winning last month's Pan Pacific Open in Tokyo, beating world number one Garbine Muguruza 6-2, 6-0 in the final.

She remains one of women's sport's biggest names and has qualified for the year end WTA Championships in Singapore.

The fact that 11th ranked former ASB Classic champion Agnieska Radwanska is returning for a second visit, indicates the line up will compare favourably to last year.

It's clear leading players want to play in Auckland ahead of the Australian Open and tour rules stating International Series events like Auckland can only have one top ten player are proving to be a real nuisance.

US Open champion Sloane Stephens shapes as a victim of that rule, indicating she would return to Auckland having been unable to defend her title this year due to injuries.

Karl Budge on the Radio Sport Breakfast

But tournament director Karl Budge is unable to guarantee her a place because he expects the world number 17 ranked American to be inside the top ten at the end of the season given she has no points to defend.

It's not impossible Stephens won't come, but unlikely. So what about the Williams sisters?

Serena intends defending her Australian Open title four months after having a baby and her plans are unknown, but given her performance on and off the court in Auckland this year, a return is unlikely.

Venus is also doubtful. She's ranked inside the top ten but could still play if she decides to not be a top ten player. The 37-year-old has the right to choose if she wants to be considered a top 10 player due to her length of service to the WTA.

If Williams was to take that option she would potentially miss out on around a million dollars in bonuses at the end of the year.

Budge is actively chasing players like US Open runner up Madison Keys and former world number one Victoria Azarenka and it would not be surprising given what he has achieved to date, to see both in Auckland this summer.