Nocioni, who scored 15 points, had his 27-footer partially blocked by Smith as time expired, allowing the Hawks to move to 2-0 after wasting nearly all of a 15-point lead.
"My teammates allow me to be the energy guy and stay active," Smith said. "We do pretty well as a team when I'm out there flying around."
Smith had 12 points, nine rebounds and six blocked shots for the Hawks. He thwarted the Sixers' run when he nailed a 3-pointer in front of their bench for a 102-97 lead with 47.5 seconds left.
"You can't be scared to take them," Smith said. "I wasn't afraid to take it."
Andre Iguodala and Elton Brand had the kind of games the Sixers expected to see more off when they signed them both to $80 million contracts in 2008. Iguodala had 27 points, 10 assists and six rebounds, and Brand had 20 points and eight rebounds.
Doug Collins is 0-2 in his return to coaching after a lengthy stay at TNT.
Lou Williams, who scored 16 points, hit a pair of free throws to make it 102-99 and make the few thousand fans left hopeful of a comeback.
Trying to seal the win twice from the line in the final 15 seconds, Johnson made two of four free throws to make it 104-101 with 4.5 seconds left. Nocioni fired from the wing for the tie and wasn't even close thanks to Smith's block.
"The last thing we wanted to do was give up a 3," Hawks coach Larry Drew said. "I told the guys to switch everything so we could defend the 3-point line. Josh just made a nice play."
With three of the starting five struggling, Iguodala and the bench got the Sixers going in the fourth.
Thaddeus Young and Iguodala had back-to-back fantastic one-handed dunks off the break, and Nocioni followed with a 3-pointer to pull them to 91-85. Nocioni missed a free throw on a technical foul called on Smith.
Johnson and Horford each hit uncontested shots to push the lead back to double digits.
Iguodala rushed a 3 and missed with the Hawks up six and 2:30 left. But Iguodala bailed himself out when he scooped a loose ball off a turnover, the 19th for Atlanta, and went behind the back to Nocioni for the basket to make it 97-93.
Brand made it a four-point game with a turnaround jumper and the Hawks coughed up the ball for the 20th time.
Nocioni slammed in a miss to make it 99-97 with 1 minute left.
The Hawks had solid numbers all around: shooting 51 percent from the floor 26 of 31, from the line and 46 rebounds.
Collins and the Sixers try for win No. 1 Saturday at Indiana.
Under Collins, the Sixers have stayed close with two of last year's top teams in the Eastern Conference. They never quit after trailing by 26 against Miami in a 10-point loss, and never let the Hawks get too far away in this one.
Brand refused to feel good about a pair of close calls.
"No, no, no," he said. "You can't be happy and take positives away from losses. Especially there, we could have won."
Sixers starters haven't produced yet on either end of the floor. In the opener, Jason Kapono, Spencer Hawes and Jrue Holiday combined for eight points and did not attempt a single free throw.
Against the Hawks, the trio combined for 14 points. Kapono was scoreless in 8 minutes.
Nocioni may start against the Pacers.
"I am still trying to figure out this team," Collins said. "Nocioni is going to give us more of a physical presence out there. We're just too small out three and it's not a good mix. The way to get Nocioni out there is to start him"
Atlanta didn't have any offensive woes from its starters. The Hawks used a 10-0 run in the first and consecutive 3s from Crawford in the third to build a 15-point lead.
Iguodala started his 248th consecutive game. Only Lakers G Derek Fisher has a longer active streak. ... After a strong debut off the bench in the opener, rookie guard Evan Turner was a non-factor for the Sixers. Turner, the No. 2 overall draft pick, was the fourth man off the bench and went scoreless by missing all five shots.