Andy Murray said he has not changed since becoming world number one following his first match as the ATP's top-ranked player.
Murray made a winning start in his first outing since dethroning Novak Djokovic, brushing aside Marin Cilic 6-3 6-2 in their Group John McEnroe opener at the ATP World Tour Finals.
The Brit converted four break points to see off the Croatian in London on Monday, extending his unbeaten streak to 21 matches.
And speaking to reporters afterwards, three-time grand-slam champion Murray insisted his mentality remains the same, despite his standing atop the men's rankings.
"I haven't thought about it [becoming world number one] too much. It didn't change much for me," Murray said.
"I didn't feel any more nervous than usual. It's just quite different conditions in there than what we have been practicing in during the week. It's a lot slower than it has been.
"Obviously, playing with a packed crowd, it changes things a bit. I think maybe some of the errors at the beginning were more getting used to playing in new surroundings again."
Next up for Murray is fifth seed Kei Nishikori on Wednesday after the Japanese star eased past Stan Wawrinka 6-2 6-3.
"I didn't see the match, but Nishikori obviously must have played very well," Murray added. "I would say this is probably his best year that he's had on the tour. He's one of the best players in the world. He's been playing very well the last few months especially."
The defeat to Murray left Cilic - who will next meet Wawrinka - with a 0-4 record at the O2 Arena since first qualifying for the World Tour Finals in 2014.
"Obviously, I need to get into the rhythm on the serve," said Cilic. "I think I wasn't hitting the spots so well today. I think obviously Andy was returning really well.
"I have to prepare definitely a little bit better [against Wawrinka] than for today. Hopefully I can start a little bit better in that match."