- WTA Finals begin in Singapore
- No.1 seed Kerber wins opener
- Straights sets win for Halep
- Injured Serena Williams sitting out event
(CNN)Angelique Kerber had to battle from a break of service down in the deciding set to beat Slovakia's Dominika Cibulkova in their WTA Finals opener Sunday in Singapore.
The world number one and top seed was stretched to the limit by the diminutive Cibulkova before winning 7-6 2-6 6-3 in their first match in the Red Group.
Earlier, Romania's Simona Halep impressed with a straight sets, 6-2 6-4, victory over Madison Keys of the United States in the same group.
Serena Williams is sitting out the eight-woman end of season finale due to injury, making Germany's Kerber the favorite to cap a super season with victory in the $7 million tournament.
But after taking the first set on a tiebreak, by seven points to five, Kerber realized she was in a battle as her seventh-seeded opponent hit back with inspired play.