INDIANAPOLIS -- The Pittsburgh Steelers spent most of Sunday afternoon playing ugly football in Lucas Oil Stadium.

But in the end, kicker Chris Boswell made it a productive day for the AFC leaders.

Boswell's 33-yard field goal as time expired rallied the Steelers to a 20-17 victory over the Indianapolis Colts.

Ben Roethlisberger's 32-yard pass to Antonio Brown to the Colts' 18-yard line with 35 seconds left set up Boswell's winning kick after he had missed a 37-yarder with 6:17 remaining that hit the right upright.

Pittsburgh (7-2) rallied from a 17-3 third-quarter deficit, finishing the game on a 17-0 run.

"We were tested, and tested in a lot of ways," Steelers head coach Mike Tomlin said. "Game circumstances kind of do that to you, but the guys didn't blink with it. They are to be given credit for that.

"We didn't play great. That is one of the reasons we were in some of those circumstances. We have to be better on offense, and we have to tackle better on defense. We were fortunate to win giving up those two touchdown passes. There's a lot of negativity, but we will deal with that negativity as part of victory. That is my preference. Guys delivered in critical moments."

The Steelers tied it at 17 with 11:52 remaining in the fourth quarter on Roethlisberger's 7-yard touchdown pass to tight end Vance McDonald and 2-point conversion pass to Martavis Bryant. That score came after linebacker Ryan Shazier's interception of a Jacoby Brissett pass at the Colts' 10-yard line.

"It was a hard-fought battle to say the least," Indianapolis head coach Chuck Pagano said. "We knew exactly what we were getting into. We played our tails off. It's not about toughness. It's not about want-to. It's not about being physical. We played really good football for a long time. Then, we made some critical, critical mistakes."

Pittsburgh sliced the deficit to 17-9 with 7:47 left in the third quarter on Roethlisberger's 7-yard touchdown pass to JuJu Smith-Schuster. The extra point was blocked.

"We didn't play very well in the first half, but we got it together in the second half, thanks to Ben and our defense," Smith-Schuster said. "When I was a kid, I loved watching Ben lead a bunch of second-half comebacks. It was great to see."

Indianapolis had extended its lead to 17-3 with 12:24 to play in the third quarter on a 60-yard touchdown pass from Brissett to Chester Rogers.

"We have to figure out a way to finish games," Colts tackle Anthony Castonzo said.

The Colts (3-7) took a 10-3 halftime lead, getting a 60-yard touchdown pass down the left sideline from Brissett to Donte Moncrief with 14:52 remaining in the second quarter and a 48-yard field goal from Adam Vinatieri with 5:51 left in the half for a 10-0 lead.

Pittsburgh finally got on the board with 30 seconds remaining in the half on Boswell's 41-yard field goal.

The Steelers were held to a season-low 100 first-half yards, with Roethlisberger going 7 of 15 for 72 yards with one interception during the first 30 minutes.

Brissett, on the other hand, was 9 of 13 for 130 yards and one touchdown before halftime.

NOTES: Pittsburgh WR Darrius Heyward-Bey left the game in the first quarter with a left foot injury and did not return. ... Steelers CB Joe Haden suffered a left leg injury in the first quarter and did not return. Head coach Mike Tomlin said after the game it was a broken fibula. ... Pittsburgh has won in Indianapolis two consecutive seasons.