With the game tied at 99 with 1.8 seconds remaining, Pero Antic passed to Horford, who hit a short fallaway as the buzzer sounded. A video review confirmed the shot beat the buzzer.
Paul Millsap and Kyle Korver had 16 points for Atlanta. Horford grabbed 15 rebounds.
Trevor Booker led Washington with 24 points and 14 rebounds.
The Hawks took their 11th consecutive home win in the series.
Korver’s 3-pointer gave Atlanta a 95-93 lead in overtime. After Jeff Teague had two baskets for Atlanta, each answered by Booker, Washington called a timeout with 11 seconds remaining in the extra period. John Wall’s penetration and pass set up Booker’s tying layup with 1.8 seconds remaining.
PACERS: In Indianapolis, Lance Stephenson had 20 points and 11 rebounds to help Indiana improve its franchise-record home start to 11-0 with a 99-94 victory over the Charlotte Bobcats.
It was the Pacers’ first game since defeating the two-time defending champion Miami Heat on Wednesday night in rematch of last season’s Eastern Conference Finals.
The Bobcats had their losing streak in Indiana extended to eight games. Charlotte was trying to win at Indiana for the first time since 2008.
NETS: Center Brook Lopez was out of the lineup for Friday night’s game at Detroit with a sprained left ankle.
Lopez is averaging 20.5 points per game, but he landed awkwardly and reinjured his ankle in Thursday’s win over the Los Angeles Clippers.
Reggie Evans replaced Lopez in the starting lineup. The Nets were also without Andrei Kirilenko because of back spasms, and Jason Terry, who has a bruised left knee. The Pistons were missing Will Bynum, who has a left adductor strain.
WARRIORS: Backup center Jermaine O’Neal underwent arthroscopic surgery for a torn ligament in his right wrist.
The Warriors said Dr. Jeff Yao operated Friday at the Stanford Orthopaedic Surgery and Sports Medicine Clinic. The team gave no timetable for his return.
O’Neal wrote on Twitter: “I am NOT out for the season contrary to some of the media reports.”
The 35-year-old is in his 18th NBA season and has had previous knee and wrist operations. He has appeared in 17 games this season, averaging 6.2 points, 4.5 rebounds and 18.6 minutes.
O’Neal is the third center behind starter Andrew Bogut to have surgery this year.
Festus Ezeli is expected to be out until at least late January following right knee surgery. Ognjen Kuzmic will be sidelined at least another four weeks after fracturing a bone in his right hand.