SINGAPORE (AFP) - Top seed Serena Williams survived a stern test from Ana Ivanovic and eased concerns over her injured knee as she opened the WTA Finals with a 6-4, 6-4 win on Monday.

The world No. 1 settled down after a nervous start to put in an ultimately clinical performance in Singapore in which she broke Ivanovic at crucial moments in both sets.

The 33-year-old also proved her fitness, in question after she withdrew from this month's China Open, by chasing down and sliding into points and at one point doing the splits.

"No one needs to drop-shot against me any more," she said afterwards. "I was just trying to fight as hard as I could, as you could see. I think I've been losing my voice from saying so many 'Come-ons'."

Williams recovered to earn the first break, but Ivanovic saved more break points at 4-1 before a net cord helped her break back and win three games in a row for 4-4.

However, the Serb wasted another break point when she volleyed long and as she served to stay in the set, Williams made her pay when she grabbed her second break and the first set 6-4.

An attritional second set came alive in the seventh game when Ivanovic's baseline smash winner was answered by Williams' fabulous cross-court drop-shot as the American went 4-3 ahead.

And with Ivanovic serving at 4-5 down, Williams again raised her game as she drilled a backhand down the line before converting the first of two match points when the Serb went long.

Williams is trying to become the first woman to win the season finale three times in a row, as well as secure her fourth year-end world No. 1 ranking ahead of Maria Sharapova.

Ivanovic, a former world No. 1 herself, is playing the year-ending tournament for the first time since 2008.

Monday's second round-robin match in the eight-woman event features Romania's Simona Halep against Wimbledon finalist Eugenie Bouchard.