After ascending to 37-33 last week, their best record in four years, the Sixers dropped two straight before this win.
Smith was sensational with an array of jumpers, 3-pointers and several rim-rattling dunks. The 6-foot-9 forward was 13-of-15 from the field while hitting all three of his 3-pointers.
Flip Murray added 19 and Joe Johnson had 11 for the Hawks (43-32), who haven't won a road game over a team currently in playoff position since a victory on Feb. 11 at Detroit. Still, Atlanta holds the No. 4 spot in the postseason.
The Hawks cut a 16-point deficit in the third quarter to 80-79 on a driving layup by Smith with 8:27 remaining. Atlanta didn't make another field goal, going 6-for-6 from the free throw line.
Philadelphia responded with a 14-2 run, keyed mostly by its bench, to take a 94-81 advantage with 4:04 left. Lou Williams, Theo Ratliff and Speights had four points apiece during the spurt.
The Sixers were dealt an early blow when starting forward Thaddeus Young limped off the court with 2:15 left in the first quarter. Young landed on Smith's foot and stayed on the court for a few minutes writhing in pain.
Young, who had played in every game this season, scored six points before the injury. He entered the game averaging 15.3 points. X-rays were negative and Young's status for Thursday's game against Milwaukee is not immediately known.
The Sixers led 49-33 late in the first half before cruising to a 51-37 advantage at the break.
Johnson and Mike Bibby combined to shoot 0-for-10 in the first half.
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