Defending champion Serena Williams survived a spirited challenge from good friend Caroline Wozniacki to advance to the final of the BNP Paribas WTA Finals Singapore presented by SC Global, defeating the Dane 2-6, 6-3, 7-6 (8-6) at the Singapore Indoor Stadium on Saturday.

Just a day earlier, the duo were spotted watching a concert by pop diva Mariah Carey together at the nearby National Stadium. Back on the tennis court, however, both cast aside their strong friendship to dish out unstoppable serves and jaw-dropping returns to try and beat each other.

Wozniacki, the world No. 8, showed immense counter-punching tenacity to keep herself in the match despite Williams' thunderous stroke-making, frustrating the American in the first set to the point where Williams smashed her racket in anger.

While Williams maintains her bid to retain the singles title, defending doubles champions Peng Shuai and Hsieh Su-wei were also triumphant in their semi-final tie against Alla Kuryavtseva and Anatasia Rodionova. The duo won 6-1, 6-4 in their early afternoon tie on Saturday.