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 one-shot lead over David 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 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 Mediate, Rod Pampling 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 U.S. Open, the only major championship he hasn’t won. But that missed cut on the PGA Tour left him with the possibility of not playing the next two weeks heading into the Safeway Open, which would be his final tune-up for Winged Foot.