His focus is on three more days of golf.
Mathis matched his best round ever on the PGA Tour, shooting 5-under 65 Thursday to grab the lead at the St. Jude Classic. The 37-year-old North Carolina native is only in his second year on the PGA Tour and had to play his way back by finishing 13th on the Nationwide Tour money list last year.
He jumped to the top of the leaderboard late in the first round, finishing with six birdies and one late bogey. He hadn't shot 65 on tour since 2009 when he went that low three different times.
"I don't feel stressed or uncomfortable in the position," Mathis said. "I know there's tons of golf left. I'm just trying to keep poise about me I guess I would say while I play and recognize that, 'Hey, there's lots of holes to play between now and Sunday,' and so just take it for what it is. Be thankful for a good round and try to reproduce it for the next three days."
Robert Karlsson, who lost a four-hole playoff here a year ago, shot 66 and is tied with John Merrick, Kris Blanks, Colt Knost and Aiken's Kevin Kisner. Knost had the only bogey-free round of the day.
Defending champion Lee Westwood opened with 69.
Patrick Reed, who helped Augusta State win its second consecutive NCAA national championship last weekend, shot 71 in his pro debut.
Mathis turned pro in 1997 after leaving Campbell University. He started playing mini tours and was on the Canadian tour before switching to the Nationwide series.
He first joined the PGA Tour in 2009 when he posted a pair of top-25 finishes, including a tie for 23rd in Memphis.
But he wound up back on the Nationwide Tour last year. St. Jude, sponsored by FedEx, is his 14th event on the PGA Tour this year and No. 45 for his career.
"Somebody asked me the other day, 'How long you been playing? Like 13 years or something like that?' I was like, 'Really?" Mathis said. "So anyway obviously the time flies, but you know it's been an enjoyable ride coming from that ... makes me appreciate playing out here for sure."
Mathis, who is from Winston-Salem, N.C., credited a visit with his coach Patrick Kelley this week for helping straighten out his line when putting. He noticed the difference almost immediately when he started on the back nine and rolled in a 9-footer to save par.
He made a birdie putt on the par-3 No. 11 from just inside 19 feet, plus 8-footers for birdies on Nos. 13 and 16. He converted a 15-footer for birdie on No. 2 to join the pack tied at 4-under.
Mathis had the lead to himself when he hit a wedge on the par-5 No. 3 to within 13 feet, then made another birdie. He padded his advantage with a 4-footer on No. 5 for his sixth birdie.
Kisner started this tournament with only his fourth round in the 60s this year.
The Aiken native, who went to the University of Georgia, hasn't finished better than a tie for 39th.
"The courses are much more difficult than the Nationwide," he said. "You really have to know where you're going to miss it and places to be. It's all just part of learning. It's why they call you a rookie."