So the Suns' 100-94 victory on Wednesday was an uphill affair, with Phoenix relying on balance when its trademark energy flagged.
''Coming off the back-to-back we were a little tired, a little exhausted,'' the Suns' Channing Frye said, ''and I think we had some mental breakdowns, but other than that, I think we just battle and grind it out.''
Six Phoenix players hit double figures, led by Markieff Morris' 18.
''It's tough on back-to-back games, especially last night when we played overtime and we got home late,'' Dragic said. ''It's just sometimes your legs don't react as fast, you think 'I have to do that' but your legs think kind of slow. In the end, when it was time to pick it up, we did it and we did our jobs.''
Markieff's twin brother, Marcus, scored 10 of his 11 in the second half, including a 32-footer at the third-quarter buzzer. P.J. Tucker had 10 points and 11 rebounds for the Suns, who took the lead for good with a 17-0 second-half run, then used free throws to hold off the Celtics at the end.
''We made some silly mistakes at the end of the game,'' Phoenix coach Jeff Hornacek said, ''but we survived, so we'll take that.''
Rajon Rondo had 18 points and 10 assists for the Celtics. Brandon Bass also had 18 and Jeff Green added 17 points.
Despite its fatigue, Phoenix had a 30-8 advantage in fast-break points.
''You can't make errors against them because they are coming, they are coming fast,'' Celtics coach Brad Stevens said. ''For all of us from the Northeast that traveled down here, we can agree that that team pushes it harder than any team we have seen all year, even more so than we saw on film.''
The Suns took the lead for good by scoring the final 12 of the third quarter and first five of the fourth, going up 71-61 on Marcus Morris' dunk with 9:45 to play.
Boston, though, came back, and Kelly Olynyk's 3-pointer from the corner cut Phoenix's lead to 88-86 with 3:59 to play. Phoenix made four of six free throws to make it 92-86, then Gerald Wallace's two free throws sliced the lead to 92-88.
Frye made a 3, the Suns' first field goal in 5:07, and Phoenix led 95-88 with 1:20 to play.
Jeff Green's two free throws with 30 seconds to play cut it to 97-94. Dragic missed a reverse layup and Boston got the ball with 23 seconds to go. But rather than try for a game-tying 3, the Celtics went for two. Rondo missed a driving layup and Jared Sullinger couldn't score on the rebound, then Gerald Green put it away with two free throws with 10.9 seconds to go.
The Celtics outscored Phoenix 16-4 to start the second half to lead 66-56, but the Suns got the last dozen of the quarter, capped by Marcus Morris' long one at the buzzer.
''Tonight the third quarter was like our best starts coming out in the third quarter,'' said Bass, who made three straight jumpers to start the second half. ''At the end of the third, they made their run. They did what they do best. They got into transition.''
The Suns never trailed in the first half but were up only 52-50 at the break.
In the first half, the Celtics scored 18 points off 14 turnovers, the Suns just two points off eight Boston turnovers.
Phoenix had only four turnovers in the second half to Boston's six.
Notes: The Celtics opened a four-game road trip. ... Gerald Green's career-high 36 points at Denver were the most by a Phoenix player this season. ... Phoenix matched its season best of 11 games above .500. ... Boston fell to 4-19 against the Western Conference. ... Bass, who didn't score in the fourth quarter, fell two shy of his season high. ... Sullinger limped off with a twisted ankle with 10.9 seconds to play. ... Phoenix is home against San Antonio on Friday night.