Guard Goran Dragic had 17 points and forward Marcus Morris added 14 for the Suns, who have won six of their last seven.
"We never quit. When we got down 14 at halftime we talked about how we weren't playing with the right effort," Markieff Morris said. "Everybody loses games but it's embarrassing to be down by (21) in the first half."
They did something about it, outscoring Denver 44-23 over the final 17 minutes.
"I know it's still early but you can't let games slip like this," guard Randy Foye said.
The Suns' surprising start to the season included a 114-103 win over Denver in Phoenix on Nov. 8. They looked like easy pickings when they fell behind 60-39 in the second quarter Friday before stunning the Nuggets in the fourth.
"We sensed we could win the game, even though we were down 21," Green said. "There was no point where we thought we were going to lose. We knew something had to change and it did."
Green got the Suns close in the third quarter when he hit four of his six 3-pointers and the Morris brothers grabbed the baton in the fourth. Markieff and Marcus both had three-point plays and a 3-pointer to get the Suns within two, and guard Ish Smith hit a fast-break layup to tie it at 88.
"Obviously our bench guys Gerald, Markeiff and Marcus helped," Suns coach Jeff Hornacek said. "We got a big lift from Ish. A lot of credit to our bench guys."
guard Randy Foye missed a 3-pointer from the top of the key and the Suns closed it out.
"Tonight, we were just a front running," Nuggets coach Brian Shaw said. "Everything, when it was going good for us in the first half, we were all smiles and played as if the game was over and didn't have a second half to play. It's my fault. I mismanaged the game."
It looked like Denver would cruise to the win when it led 76-59 in the third, but the Suns chipped away, with Green leading the way. His fourth 3-pointer of the third made it 80-72.
NOTES: Nuggets F Jordan Hamilton missed Friday's game while serving a one-game suspension for throwing a punch at Oklahoma City C Steven Adams in Denver's loss to the Thunder on Tuesday. ... Suns F Channing Frye entered Friday having scored 20 and 22 points in his last two games. It is the first time he scored at least 20 points in consecutive games since February 2011. He was scoreless at halftime against Denver. ... Nuggets G Ty Lawson played despite having the flu Thursday. He was excused from practice and felt better by Friday's shootaround. ... Phoenix Gs Goran Dragic and Eric Bledsoe are both averaging more than 18 points and six assists a game. The last teammates to do so were Michael Jordan and Scottie Pippen of the 1991-92 Chicago Bulls.