SINGAPORE (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 number four 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 number one 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.
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 at a courtside.
"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."
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 number one 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.
Editing by Martyn Herman and Julian Linden