- Maria Sharapova beat Italy's Sara Errani in straight sets to win opening match in White Group
- Defending champion Petra Kvitova makes 41 unforced errors as she is beaten by Agnieszka Radwanska for first time
- Serena Williams returns to action for first time since U.S. Open win, beating Germany's Angelique Kerber
Maria Sharapova kept alive her hopes of finishing the year as world No. 1 when beating Italy's Sara Errani 6-3 6-2 in her opening match of the WTA Tour Championships in Istanbul.
The Russian, who won the event in 2004, has called attaining the status her 'biggest priority this week' at the season-ending tournament.
The 25-year-old needs to win at least two matches in Istanbul while hoping that Victoria Azarenka, the Belarusian who currently holds the title of the world's best, slips up in the Red Group if she is to finish the year on top.
Sharapova, whose victory margin was identical to her only previous meeting with Errani, who she beat in the final of this year's French Open, was delighted to make an impact at the WTA Championships after injury forced her early withdrawal last year.
"This is such a great feeling, not only to make it here but to feel good physically," the World No.2 told the WTA's official website.
"When you're out here and feeling good it's such a big motivation, and it's the last tournament of the year too, so while I'm going to enjoy some time off after this, right now it's all business."
Sharapova is competing in the White Group, where Poland's Agnieszka Radwanska earned her first career win against Petra Kvitova at the fourth attempt as the defending champion lost the opening match of the championships.
The Wimbledon runner-up won 6-3 6-2 in a match where the Czech, looking to become the first woman to win back-to-back titles since Belgium's Justine Henin in 2007, produced 41 unforced errors in contrast to just five from her opponent.
"I was not very comfortable on court and I didn't feel pretty well," said Kvitova, who cut a dejected figure afterwards.
The only Red Group game of the day found Serena Williams playing her first match since winning the U.S. Open in August and she maintained her winning run as she beat Angelique Kerber, the German making her debut at the event for the world's top eight players.
The Wimbledon, U.S. Open and Olympic champion won 6-4 6-1 against the left-hander who had inflicted Williams' only defeat in her 27 matches preceding the tie, with Kerber winning in Cincinnati in August.
The victory, which quickly reached its conclusion once Williams broke for 3-1 in the second set, was greeted with huge enthusiasm by those watching inside Istanbul's Sinan Erdem Dome.
"I've never been here but I never knew I had so many Turkish supporters," Williams told wtatennis.com.
"It was really an honor to hear the fans go crazy and be really loud when I walked out. It made me have this smile on my face, and an even bigger smile inside."
On Wednesday, Williams is scheduled to play first against Li Na, the former French Open champion from China, while Azarenka enters the action when taking on Kerber, who broke into the top five in the rankings for the first time this week.
"Every match here will be tough - I obviously have great champions in the group," said Azarenka. "It's challenging, definitely, but I'm looking forward to it."