Angelique Kerber was given a brutal reminder of the extra burden she carries as the new world number one 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.