Singapore - Angelique Kerber was given a brutal reminder of the extra burden she carries as the new world No 1 when she was forced to battle through three sets before winning her opening match at the WTA Finals against Dominika Cibulkova on Sunday.
The German was pushed all the way by an inspired Cibulkova before the Slovakian finally buckled and Kerber prevailed 7-6 (7-5) 2-6 6-3.
Although it was not her most polished performance, Kerber's gritty victory underlined her fierce determination to cap her breakthrough season by adding the prestigious championship to her Australian Open and US Open titles.
"Of course the pressure is there," Kerber said. "I feel the pressure of the different situation and I also have a lot more things to do in the last few days.
"It's a challenge for me and I'm trying to play the best tennis in the last tournament in the last week of the year.
"I'm a little bit relaxed now. It was a really good, tough match so I think that was really important for me to have a match like that."
Simona Halep, who looms as one of Kerber's biggest threats, also laid down an ominous marker as she demolished rising American Madison Keys 6-2 6-4 in the first match of the $7 million tournament.
A finalist two years ago, the Romanian looked in great form on her return to Singapore's purple indoor hardcourt, needing just over an hour to win her round-robin contest with the big-hitting Keys.
The American admitted to feeling some anxiety in her first appearance in the elite eight-player event as she struggled to find her rhythm against Halep, one of the most tenacious counterpunchers in the game.
"There were definitely some nerves today, which I don't think is surprising," Keys said.
"It's been a while since I've gone to a tournament where it's all felt new...so that was kind of difficult to deal with."
Despite being in control, Halep also had a brief moment of self-doubt in the second set, calling for her coach Darren Cahill, who told her just to relax and stick to her gameplan.
"I panicked a little bit because I missed some easy shots," Halep said.
"He knows how to talk to me when I need. He just calms me down and it's perfect."
There was no obvious sign of the intense fight that was to come when Kerber clinically broke Cibulkova in the opening game of their match and comfortably held her own serve.
But the seventh-seed Cibulkova clawed her way back, forcing a tiebreaker than dominating the second set, rattling Kerber with her penetrating groundstrokes into the corners.
Cibulkova seemed to have gained the ascendancy when she broke Kerber's serve early in the deciding third set but the world number one dug deep and reeled off five of the last six games to clinch victory and remain on course for next weekend's semifinals with the top two players from each group advancing.
"It is a really, really tough one. It was really close," a disappointed Cibulkova said.
"In this game, it's all about small details. She was just bit more consistent in few games, and that's what made her win."
The four players in the White Group, the defending champion Agnieszka Radwanska, US Open runner-up Karolina Pliskova, French Open champion Garbine Muguruza and last-minute qualifier Svetlana Kuznetsova, all play their opening matches on Monday.
Results from the WTA Finals tournament in Singapore on Sunday (x denotes seeding):
Round-robin stage Red Group
Simona Halep (ROU x3) bt Madison Keys (USA x6) 6-2, 6-4
Angelique Kerber (GER x1) bt Dominika Cibulkova (SVK x7) 7-6 (7-5), 2-6, 6-3