- Tennis star will not play in Singapore's WTA finals due to shoulder injury
- She also withdrew from the event in 2015 over health concerns
- Hasn't played since US Open
- Angelique Kerber locks down year-end No. 1
(CNN)Serena Williams will not take part in the WTA Finals in late October due to a shoulder injury, meaning she is skipping the elite year-end event for the second straight season.