Spain's Garbine Muguruza and Karolina Pliskova of the Czech Republic both claimed victories on the first day of action at the Women's Tennis Association Finals in Singapore.
Muguruza, the reigning Wimbledon champion, played another Grand Slam winner, Latvia's Jelena Ostapenko, at the Singapore Indoor Stadium in the end-of-season tournament featuring the world's top-ranked female players.
The Spaniard, though, brushed aside the French Open winner 6-3, 6-4 to join Pliskova at the top of the White Group standings.
"She’s a very aggressive player," Muguruza said.
"Ostapenko had an incredible run this year, and she has the confidence, that’s what makes her even more dangerous.
"I knew going into the match that she could play unbelievable, so I kind of went in prepared."
Czech world number three Pliskova claimed a comfortable 6-2, 6-2 victory over Venus Williams of the United States.
Pliskova was world number one for seven weeks in the summer and dominated Williams, the oldest player at this year's tournament at 37.
Williams has appeared at the end of season event on five occasions and won the title in 2008.
The Red Group is due to begin tomorrow, with seven of the eight finalists still in position to finish the year as world number one if the results go their way.
Muguruza, top of the rankings for three weeks this year, is 40 points behind current leader Simona Halep.
The Romanian will play France's Caroline Garcia tomorrow.
It will be followed by Ukraine's Elina Svitolina against Caroline Wozniacki of Denmark.