Australian David McKenzie has shot a nine-under 62 to trail Phil Mickelson by a shot as the American golfing superstar made his senior debut in Missouri.
Mickelson, a five-time major champion, decided to make his over-50 debut this week after missing the cut at The Northern Trust and getting eliminated from the PGA Tour's FedEx Cup playoffs.
He finished with 11 birdies and shot a 10-under 61 to lead by one stroke from McKenzie.
"It's a lot of fun. It's a fun environment. And it's fun to see a lot of the guys I grew up watching, and played with them for a number of years, and played a bunch of Ryder Cups and Presidents Cups with a lot of guys out here," Mickelson said.
"I was a little nervous, too, because I wanted to come out and play well. I was playing really well."
Australian Rod Pampling was in a group tied for third at seven-under with South Korean KJ Choi, Rocco Mediate and Tim Petrovic.
Ernie Els, Bernhard Langer and Vijay Singh were among those another shot back.
"We have a lot of golf left," said Mickelson, who hit all but one green in regulation. "A lot of players went really low. The quality of golf out here is really impressive."
Mickelson, who turned 50 in June, was optimistic that his game was rounding into shape for the US Open, the only major championship he hasn't won.
McKenzie made back-to-back birdies to finish the front nine, then rattled off three in a row on the back to shoot 62 on the Ozarks National course and finish the day in outright second place.
"I made a lot of putts today, which makes it a lot easier," he said.
"It has a little bit of a feel like Australia.
"I think it's Bill Coore, Ben Crenshaw that designed it, so I just like the way that it sits and feels. It's a really comfortable golf course."