Playing the event at TPC Scottsdale for the 10th occasion, Haas bounced back from a recent run of indifferent form to record a round of 7 under par 64, his equal best of the 30 rounds he has played at the famous desert course.
Joining Fowler in the chasing pack at 5 under are Billy Horschel, Bryson DeChambeau, Chesson Hadley, Gary Woodland and Chris Kirk.
World number two Jon Rahm is well placed at 4 under despite a bogey at the driveable par four 17th.
The Spaniard received a lucky break at the 2nd when his approach from 90 yards hit his playing partner Xander Schauffele's ball on the fly and stayed within reasonable distance of the hole. Schaufelle's ball ricocheted off the back of the green but was replaced.
A win this week would move Rahm past Dustin Johnson to the top of the world ranking and he is well placed to challenge for a third PGA Tour title.
Jon Rahm: Lucky or unlucky at the par-4 2nd?
Two-time defending champion Hideki Matsuyama began with a round of 68, Jordan Spieth struggled with an opening 72 to be in 83rd position while Justin Thomas is well placed after a 68.
Greg Chalmers leads the Australians after his round of 68 although a bogey at the last took some gloss off his day.
Cameron Smith and Marc Leishman each began with 69, Aaron Baddeley had 72 while Geoff Ogilvy was unable to recover from a horror start when 5 over through five holes and finished with 74.
FAVOURITE COURSE IN AUSTRALASIA?
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