Dominika Cibulkova and Carla Suarez Navarro were straight-sets winners at the start of the season-ending WTA Tournament of Champions on Tuesday.

Second-seeded Cibulkova of Slovakia defeated eight-seeded Tsvetana Pironkova of Bulgaria 6-3 7-6 (8-6) in round robin play in the Sredets group.

Cibulkova was down 3-2 in the first set but won the next four straight games and the set. Pironkova tried to come back in the second by going up two breaks at 4-1, but the Slovak hit back and won 12 of the next 13 points with the set eventually being decided in a tie-break.

Cibulkova said she was happy with her game because of “the right tactics,” but added that “everyone who is here is a good player, so I just have to keep doing the right thing to keep winning.”

In the group’s other match, Spain’s Suarez Navarro overcame fourth-seeded Andrea Petkovic of Germany 6-0 6-4.

The Spanish star needed just 30 minutes to take the first set without surrendering a game mostly thanks to her outstanding one-handed backhand. In the second, she broke twice for a 5-2 lead before Petkovic rallied for a comeback, getting to 5-4, but Suarez Navarro served out to seal the set at 6-4.

“The first match is never easy, and I’m happy with my performance,” Suarez Navarro said. “I hope to have a good week.”

In the Serdika group opener, Italy’s Flavia Pennetta, seeded third, eased to a 6-1 6-2 victory over France’s Alize Cornet, who had to take a medical time-out because of an injured finger.