A birdie-birdie-birdie finish saw him card 67, then propelled him to the top of the leaderboard on 14 under.
Japan's Hideki Matsuyama's bid for a rare hat-trick was over before the second round began, as the world No. 5 withdrew from the Phoenix Open on Friday prior to hitting a shot with a left wrist injury. "It was anything but that", the five-time major champion said. He was tied with reigning Rookie of the Year Xander Schauffele (66) and Daniel Berger (68).
The Phoenix Open always boasts one of the most party-like atmospheres on Tour, and Rahm appreciates the change of pace.
Fowler finished second here in 2010, as well, and finished T4 previous year. "Just a little off in certain areas". "We took advantage of the last few holes, but that is the biggest scoring opportunity on the golf course".
The other British player to survive the cut, Scotsman Martin Laird, overcame a double bogey at the first to post five birdies in seven holes on his inward nine and he shares 14th at 10 under.
DeChambeau birdied the final two holes, while Fowler bogeyed the 17th but was helped by a run of four birdies in six holes on the front nine, including a stunning approach to the sixth.
"It's going to take a lot of birdies (to win) out here".
He said he'd try to shut out thoughts of the number one ranking, which he admitted got to him in a disappointing finish at Torrey Pines last week.
"I haven't been playing very well", he said.
The 47-year-old is seeking his 43rd PGA Tour title, and his first title since the 2013 British Open.
"For me to not give away shots off the tee the way I have in the past has been very beneficial", said Mickelson, who had five birdies without a bogey, finishing birdie-birdie.
Rahm ended his round well with four birdies in his last six holes, while Reavie had a birdie-birdie finish.
Playing partner and fellow former Arizona State University graduate Mickelson, the 47-year-old three-time victor who has dropped to 49 in the world rankings, did not make a bogey and birdied the final two holes.