Just about the only poor shot Phil Mickelson hit in his PGA Tour Champions debut was a wayward iron off the tee.

He still turned it into a birdie.

It was one of five straight on the back nine Monday for the five-time major champion, who 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.

Mickelson finished with 11 birdies and shot a 10-under 61 to take a 1-shot lead over David McKenzie in the Charles Schwab Series at Ozarks National.

“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 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.”

K.J. Choi, making his second start on the PGA Tour Champions, was joined in a group at 7 under by Rocco MediateRod Pampling and Tim PetrovicErnie ElsBernhard 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.”