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

In the second match, Romania's Simona Halep joined Williams at the top of the Red Group standings after the world No. 4 brushed aside Canada's Eugenie Bouchard 6-2 6-3 on the purple hardcourt at Singapore's Indoor Stadium.

Illness and a knee problem forced the American world No. 1 to withdraw from her last two tournaments since winning a third straight U.S. Open title last month and Williams struggled early in an arena hosting the event for a first time, but fought on admirably for the win.

The victory was Williams' 16th in a row at the tournament as the 33-year-old American remains on course for a hat-trick of titles as she attempts to retain a No. 1 ranking under threat from Maria Sharapova this week.

Two of the WTA's most-improved players of the last year faced off in the second match but an expected tight encounter failed to materialise as Halep blew away an erratic Bouchard to open her account in her first appearance at the event.

Halep, 23, raced to a 4-0 lead in the opener and although Bouchard finally got on the scoreboard after 21 minutes, the damage was already done and the French Open finalist was able to seal the set as her opponent's unforced error count grew.

Bouchard, 20, could barely find the court with her attempted winners as Halep continued to hound her at every opportunity and it was somewhat fitting that a double fault on match point handed victory to the Romanian in just 68 minutes.

"I played really well and am so happy that I was able to find my game again. I stayed aggressive and got plenty of winners but she's a great player, very young and has a big future ahead of her," Halep said.