SOFIA, Bulgaria - Dominika Cibulkova, Carla Suarez Navarro and Flavia Pennetta 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 (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."

Suarez Navarro's win was her 50th WTA match win of the year, making her only the third player in 2014 to reach that mark after Ana Ivanovic (58 wins) and Serena Williams (52).

In the Serdika group opener, Italy's 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.

"I was really focused and consistent," Pennetta said. "I'm very much looking forward to my next match on Thursday."