Serena Williams serves a shot to Ana Ivanovic during the WTA Tour Finals in Singapore on Monday. Photo: CFP

Serena Williams shook off the rust and a feisty challenge from Serbia's Ana ­Ivanovic to open the defense of her WTA Tour Finals title with a 6-4, 6-4 victory in the opening match of the elite eight-player tournament on Monday.

Illness and a knee problem forced the American world No.1 to withdraw from her last two tournaments since winning a third straight US Open title at the start of September and Williams struggled early on the purple hard court in Singapore.

Pre-match pyrotechnics left the ­Indoor Stadium looking as hazy as the city-state's streets on a polluted day but once the smoke had cleared, Williams was forced to dig deep in her first service game, staving off two break points before holding.

Ivanovic, making her first appearance in the season-ending event since 2008 when it was held in Qatar, had no such problems on her own serve in the early skirmishes but once Williams found her range the American broke and held to love to move 4-1 ahead.

The Serb overcame two double faults in her next service game to hold and then started to mix up her strategy, refusing to go toe-to-toe with her powerful opponent with her drop shots causing Williams all sorts of problems.

A lucky net cord brought up three break points for Ivanovic in the seventh game and she only needed one of them as Williams double-faulted to hand back her early break.

With the crowd cheering her on, Ivanovic suddenly had the momentum and she began to make Williams scurry all over the court and even had a break point to move 5-4 ahead before her opponent dug deep to stave off the scare.

Williams saved the break point and welcomed a big forehand and a service winner with shouts of "come on!" to edge 5-4 ahead and put the pressure back on Ivanovic, who surrendered the set as her service fell apart under pressure.

The pair traded breaks to open the second set and then raced through their service games until Williams made the vital breakthrough in the 10th game with a series of thunderous forehands before an Ivanovic error handed the contest to her opponent on her first match point.

"I am really excited with how I played tonight. I have been working hard all my life for nights like this and I love it. So thank you for coming tonight," Williams said in a courtside interview.

The victory was Williams' 16th in a row in the tournament.

"I was really fighting really hard because Ana played so well and had so many fans here behind her as well," Williams said. "I was fighting as hard as I could and I think I am losing my voice from all of those 'come ons' I was shouting."

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.

In the night's later game, Romania's Simona Halep beat Eugenie Bouchard of Canada 6-2, 6-3.