SINGAPORE (AP) Top-seeded Angelique Kerber struggled to defeat seventh-seeded Dominika Cibulkova 7-6 (5), 2-6, 6-3 on the opening day of the WTA Finals in Singapore on Sunday.
Kerber, who secured the top ranking last month, battled Cibulkova for 2 hours, 17 minutes before the Slovakian finally succumbed.
''It was a really good match from both of us, especially the first set,'' Kerber said. ''It's a good start to the tournament like this, where you knew you had to play your best to win.''
Earlier on Sunday, third-seeded Simona Halep won the first match of the round-robin portion of the tournament by taking a 6-2, 6-4 decision over sixth-seeded American Madison Keys.
In Red Group action, Kerber and Halep stand at 1-0, while Keys and Cibulkova are at 0-1.
On Tuesday, Kerber will play Halep, while Cibulkova and Keys, both making their WTA Finals debut, will play each other.
Kerber posted 29 winners and 32 unforced errors to 36 winners and 34 unforced errors for Cibulkova.
Cibulkova dropped serve in the opening game of the match, which initially allowed Kerber to establish a 4-2 lead in the first set.
Kerber didn't hold on to the advantage, surrendering her own serve on a second double-fault in the eighth game.
That sent the first set to a tiebreaker where the German eventually prevailed.
''At the beginning I was too excited, but after a few games I started to play my game,'' Cibulkova said. ''It was a really tough one, really close, and in these kind of games it's about the small details.''
Cibulkova dominated the second set, racing to 4-0 lead.
In the third, Cibulkova was temporarily in charge with a 2-0 lead, but lost five of the next six games to end up on the losing side of the result.
It's been a stellar season for Kerber, who won her first two Grand Slam titles at the Australian and US Opens, and also brought home the Olympic silver medal from Rio.
Kerber's never journeyed beyond the round-robin stage of the WTA Finals in three previous appearances in 2012, 2013 and 2015.
In the opening match, Keys played erratic tennis throughout the 69-minute contest, losing serve on four of 10 break points faced.
''I definitely think there were some nerves,'' Keys said. ''I think one of her strengths is making you feel like you have to go for more and take the risks. I think sometimes she makes me uncomfortable and I back away from playing my game.''
Keys held serve in the opening game of the match, but then saw Halep win the next five games for a 5-1 lead in the first set.
Halep lost an initial 4-2 lead in the second set, but from 4-4 won the final two games.
The Romanian reached the final here in 2014 and now holds a 5-1 head-to-head record against Keys.
''I think I played exactly what I had to play against her,'' Halep said. ''I was focused. Everything went as I wanted, so I'm happy.''
