The field of eight top women's singles tennis players has been whittled down to four at the BNP Paribas WTA Finals Singapore presented by SC Global. With the completion of the round-robin matches yesterday, world No. 1 Serena Williams, No. 4 Simona Halep, No. 6 Agnieszka Radwanska and No. 8 Caroline Wozniacki are left to contend for the Billie Jean King trophy.