Kirk took a share of the lead at the PGA Tour's season-ending Tour Championship, alongside playing partner Billy Horschel, after an opening round four-under-par 66.
The 29-year-old is assured of winning the FedEx Cup with victory at East Lake Golf Club, and despite claiming pre-tournament the money was no factor, Kirk said he was finding it tough dealing with the pressure of expectation.
"It's kind of tough to get your mind off what's going on here this week," Kirk conceded on Thursday.
"But [playing with Horschel] may have helped a little bit. I would have been comfortable with a lot of guys here.
"Billy's one of many guys that I've become good friends with and have known for a long time. There are a lot of guys out here on Tour that I played against in junior golf and then in college golf."
Kirk said the greens at the Atlanta, Georgia course were tough to play, but were worthy of being the scene of the season-ending event.
"Obviously these greens, sometimes... can get a little bit spiked up," the Georgia native said.
"But you can have a thousand people could play this golf course, all with metal spikes you wouldn't be able to tell.
"They're unbelievably good and unbelievably healthy. [It's] definitely not an issue being in the last group."
Kirk went bogey-free on Thursday, carding four birdies, but said reading the greens was his toughest challenge.
"These greens, like I said, are in perfect shape," he said.
"They are very, very tricky to read with the grain, and the ball... generally goes with the grain, but not always.
"Despite how perfect they roll, they can be tough."