Maria Sharapova and Simona Halep won the opening Red Group matches as the WTA Finals got under way in Singapore.
Halep, the top seed and world number two, thrashed US Open champion Flavia Pennetta 6-0 6-3, before Sharapova saw off Agnieszka Radwanska 4-6 6-3 6-4.
The eight-player season finale begins with two round-robin groups, the top two from each qualifying for the semis.
World number one Serena Williams is absent as she recovers from injury.
The White Group consists of Spain's Garbine Muguruza, Petra Kvitova and Lucie Safarova of the Czech Republic, and Germany's Angelique Kerber.
Halep wins in 72 minutes
Romania's Halep reeled off the first seven games on her way to beating Pennetta in just 72 minutes and taking control of the Red Group.
Halep, last year's runner-up, showed no sign of the ankle injury that hampered her earlier this month.
"I didn't play the right game against her at the US Open," she said. "Here I was well prepared and I knew how to play against her."
Pennetta, who is playing her final tournament before retiring, said: "She was playing really well today. Maybe I had not too much energy. I didn't push a lot like I always do."
Sharapova earns first win since July
The Russian last completed a match at Wimbledon in July, since when she has struggled with leg and forearm injuries, so it was no surprise that her game was far from at its sharpest.
Sharapova showed that her battling qualities remain intact, however, as she hit back from a set down to win in just short of three hours, after recovering from 15-40 when serving for the match.
"One match down," said an emotional Sharapova afterwards. "I still have a few to go but it's great to get a win against such a tough competitor as Radwanska, we always have such difficult matches.
"To be able to come out here, fight until the end and win it means a lot to me."
|WTA Finals groups|
|Red Group||White Group|
|Simona Halep (Romania, rank 2)||Garbine Muguruza (Spain, 4)|
|Maria Sharapova (Russia, 3)||Petra Kvitova (Czech Republic, 5)|
|Agnieszka Radwanksa (Poland, 6)||Angelique Kerber (Germany, 7)|
|Flavia Pennetta (Italy, 8)||Lucie Safarova (Czech Republic, 9)|