Serena Williams withdraws from WTA Finals

US superstar Serena Williams has withdrawn from the elite WTA Finals in Singapore, saying she's still working to recover from the shoulder troubles that hindered her in 2016.

The WTA announced Williams' withdrawal from the event that starts on October 23 on their Twitter feed, posting a video of Williams explaining the decision.

It will be the second straight year that Williams is skipping the season finale in Singapore due to injury.

"Hey everyone in Singapore," Williams said. "I'm really, really bummed that I won't be able to come compete this year.

"It's been a really tough year for me just dealing so much with these shoulder injuries. My doctor insists that I stay home and heal it every single day."

The 22-time Grand Slam champion earlier pulled out of two tournaments in China after being knocked out of the semi-finals of the US Open.

That defeat saw Williams' 186-week reign atop the world rankings end as Germany's Angelique Kerber reached the summit and captured the Flushing Meadows title into the bargain.

Williams claimed just one Grand Slam title in 2016, at Wimbledon. After that, shoulder troubles hampered her in Rio where her hopes of a fifth Olympic gold medal were crushed in the third round.

WTA CEO Steve Simon said in a statement that organisers were "as disappointed as the fans".

"We wish her a speedy recovery and look forward to seeing her back competing, fit and healthy," he added.

Williams' withdrawal opens up the eighth qualification spot to Britain's Johanna Konta, Spain's Carla Suárez Navarro and Russia's Svetlana Kuznetsova.

Either Navarro or Kuznetsova must win the title in Moscow at the VTB Kremlin Cup next week in order to edge out Konta for the eighth and final qualifying spot.

The tournament in Singapore will see reigning champion Agnieszka Radwanska defending her title against current top seed Kerber.