World number one Simona Halep overcame Caroline Garcia in their opening round-robin match on day two of the WTA Finals in Singapore.
France's Garcia, number eight in the rankings, had beaten Halep to win the China Open title earlier this month.
But the Romanian put in a solid performance in her first match as number one to win 6-4 6-2.
Denmark's Caroline Wozniacki and Ukraine's Elina Svitolina meet in the second Red Group match on Monday.
Wimbledon champion Garbine Muguruza and Karolina Pliskova both earned straightforward victories in the White Group on day one.
Spain's Muguruza beat French Open winner Jelena Ostapenko 6-3 6-4 while Czech world number three Pliskova brushed aside American veteran Venus Williams 6-2 6-2. WTA Finals line-up RED GROUP WHITE GROUP 1. Simona Halep (Rom)1. Garbine Muguruza (Spa)2. Elina Svitolina (Ukr)2. Karolina Pliskova (Cze)3. Caroline Wozniacki (Den)3. Venus Williams (USA)4. Caroline Garcia (Fra)4. Jelena Ostapenko (Lat)
Halep's meeting with Garcia appeared to be far more competitive in the opening stages, as the world number one was forced to fend off two break points in her first service game, which lasted 15 minutes.
Garcia, who is making her debut in the Finals, got the break she was searching for to lead 3-2 but Halep struck back immediately and broke again on her way to securing the first set.
The Romanian secured a double break in the second set and held to love for a 5-1 lead, before seeing out the match in one hour and 28 minutes.
Halep, who has yet to win a Grand Slam title, leads the WTA rankings ahead of Mugurzua, although seven of the eight finalists are able to finish the year as world number one.
"I'm really pleased," said the 26-year-old. "It wasn't an easy first match against a player that beat me two weeks ago. I felt really well out there."
