US tennis player Serena Williams has withdrawn from the WTA Finals in Singapore, saying she's still working to recover from the shoulder troubles that hindered her in 2016.The WTA announced Williams'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 herself explaining the decision.The 22-time Grand Slam champion pulled out of two tournaments in China after exiting from 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.