Singapore - Simona Halep laid down the gauntlet at the WTA Finals on Sunday, demolishing rising American Madison Keys 6-2 6-4 in the opening match of the $7m end-of-season event.
A finalist two years ago, the Romanian looked in great form on her return to Singapore's purple indoor court, needing just 68 minutes to win her round-robin match against Keys in straight sets.
The big-hitting Keys struggled to find her rhythm against Halep, who is one of the most tenacious counter-punchers in the game, committing 18 unforced errors and losing 13 points in a row to concede the opening set after just 27 minutes.
Halep broke Keys for a third time to grab the early advantage in the second set, but a sloppy service game allowed the American to draw level at 4-4.
However, Halep broke straight back and served out the match to go straight to the top of the Red Group standings.
World number one Angelique Kerber was scheduled to play Slovakia's Dominika Cibulkova in the day's other Red Group match with the two players from the pool phase advancing to next weekend's semi-finals in the elite eight-player tournament.
The four players in the White Group, Agnieszka Radwanska, Karolina Pliskova, Garbine Muguruza and last-minute qualifier Svetlana Kutznetsova, all play their opening matches on Monday.