Kelce didn't return to the game and the Chiefs struggled offensively in the second half.
The collapse of the Chiefs, who had gone into halftime with a 21-3 lead, somewhat mirrored their regular season, which began with a 5-0 start but also included a stretch of six losses in seven games. But officials ruled that the whistle had blown Mariota down.
The Titans drew to within a touchdown on a 35-yard run by Derrick Henry with 14:08 left in the fourth quarter. This score made the game a lot closer, at 21-10, with about half the third quarter left to play.
Kelce was knocked out of the game in the second period with a concussion, following an ugly head-to-head collision with a Tennessee defender. At the end of the Titans' opening drive of the second half, Mariota attempted pass on third-and-goal from the Chiefs' 6-yard line.
The Titans drove into Chiefs territory midway through the second quarter but Peters intercepted an underthrown pass from Mariota in the red zone, ending the threat.
It was his second career touchdown catch.
Dick LeBeau's defense forced yet another punt on the ensuing Chiefs possession, and the Titans embarked on yet another long scoring drive, this time going 80 yards on 11 plays and ate up just over five minutes of game time. The Titans quarterback snagged it and dove into the end zone to cut Kansas City's lead to 21-10.
The two-point conversion attempt failed, leaving Tennessee down 21-16.
That has become a familiar refrain for the Chiefs in the playoffs.
The Titans ran it with Henry, who had nowhere to go inside, bounced it outside, got a lead block from Mariota, of all people, and sprinted 22 yards for the game-clinching first down.
Chiefs cornerback Marcus Peters hid out in the locker room during the national anthem before their playoff game against Tennessee, just as he has done all season.