Roger Federer felt he put too much pressure on himself early in his straight-sets win over Tomas Berdych at the ATP World Tour Finals.
The Swiss star completed a 6-4 6-2 victory in London on Sunday to make an ideal start in Group Stan Smith.
Federer had fallen behind an early break against Berdych before recovering to win in one hour, nine minutes.
But the six-time champion and 34-year-old said he had expected too much from himself against Berdych.
"It is like a first round so you can't expect too much of yourself. And I probably, maybe, did," Federer said.
"I thought I was playing really well in practice, I've had a good year. I thought I was just going to come out and swing from the hip and feel good. And I didn't.
"I put myself under too much unnecessary pressure at the beginning, trying to go for too big of a second serve, even though I hadn't found my rhythm yet. That was a mistake and it cost me.
"Thankfully I reacted quickly and was able to dominate the baseline eventually, which is clearly really important on this surface here in London."
With his next match a clash against in-form world number one Novak Djokovic on Tuesday, Federer said that added to the pressure of the situation.
Djokovic thrashed Kei Nishikori 6-1 6-1 in an ominous performance to open the tournament.
"It's not really the conditions, it's the expectations and the pressure," Federer said.
"The sheer [pressure] of just having to win the first round of the group, especially having Novak in the group maybe creates that as well because if you beat him it's amazing, but if you don't it's what is expected in some ways, even though I've had a good run against him in previous years.
"But it was important to focus on Berdych and nothing else and now that the match is over I'm relieved, happy.
"My game's there, my mind is there, my body is ready and that's how you want to feel playing Novak in the next match."