Both players are at 16-under 126 through two rounds at the par-71 Club at Chatham Hills Course.
Johnson posted a bogey-free round and has not bogeyed a hole since a four on the par-3 second hole on Friday.
He birdied six holes in an eight-hole stretch in the middle part of the round, posting a score of 32 on both the front and back nines.
Munoz, who shot a 12-under 59 on Friday, looked like he would run away with the tournament after birdieing six of the first seven holes to reach 18 under.
He was erratic on the back nine, playing even-par golf until hitting a poor tee shot on 18 and ending up with a double bogey to fall back into a tie with Johnson.
Johnson hit back strongly after trailing his playing partner by seven shots on the front nine.
"Obviously, he got off to a great start, I got off to a slow start, and I was seven back," said Johnson.
"But you've just got to keep trying to make birdies. Get it in the fairway and just get looks. That's what I did. I did that from about 6 on in. I played really well."
Six players are tied for third place at 12 under. Of those, Poland's Adrian Meronk and Spain's David Puig posted the best rounds of Saturday, each eight-under rounds of 63.
Meronk eagled the par-5 seventh and 13th holes.
Also tied at 12 under are Thomas Pieters of Belgium, Patrick Reed, Chile's Joaquin Niemann and Sweden's Henrik Stenson. Spain's Jon Rahm, South Africa's Brandon Grace and Bubba Watson are one stroke behind that group of six in a three-way tie for ninth.
Cameron Smith slipped back into a tie for 14th at nine under after signing for a two-under 69 in the second round.
Lucas Herbert (70) and Marc Leishman (77) were at level par, one shot clear of fellow Australian Matt Jones.
- with AAP
5°C