The Hawks, who haven't made the playoffs since 1999, got back to .500 in a brutal opening stretch of games and added to their opening-night win over Dallas.
Atlanta led throughout the second half, but the Suns were right there when Nash drove for a basket with 4:42 remaining. Marvin Williams, who had 20 points and 12 rebounds, responded by hitting back-to-back jumpers, keeping Phoenix from mounting a comeback.
Smith sealed the win in the final 2 minutes, fighting hard on the offensive boards to get a couple of chances off Joe Johnson's missed jumper. The second one rolled in, and Smith was fouled by Grant Hill.
The Atlanta forward let out a yell and slapped hands hard with Acie Law, then went to the line and finished off the three-point play that gave the Hawks a 99-89 lead with 1:47 remaining.
Horford grabbed 15 rebounds to lead four Atlanta players in double figures off the boards.