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.