Roger Federer revealed that the ATP World Tour Finals creates a pressure-cooker atmosphere that can easily affect you.
The world's top eight players are in London for the ATP's swansong event and with just three games before the semi-finals are decided, the pressure is on the players from the get go.
Federer is set to face top seed Novak Djokovic – who is on a 23 match unbeaten streak – on Tuesday evening in the English captial.
The Swiss Maestro indicated that the mere presence of the indomitable Serb in his group has an impact.
"It's not really the conditions, it's the expectations and the pressure," the 34-year-old told sportal.com.au.
"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.
"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."
Federer got is campaign off to a winning start with a straight set win over Tomas Berdych on Sunday, but despite his excellent record against him, the former world number one even approached that match with some trepidation.
"It was important to focus on Berdych and nothing else and now that the match is over I'm relieved, happy," he added.
"It is like a first round so you can't expect too much of yourself. And I probably, maybe, did. 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."