KANSAS CITY, Mo. — The Kansas City Chiefs are locked in as the AFC's No. 2 seed for the NFL playoffs thanks to their 31-21 win over the Los Angeles Chargers and the Miami Dolphins' 27-24 upset at the New England Patriots.

"Hail to the Dolphins," Chiefs coach Andy Reid said.

While the Chiefs had to wait to learn what happened in the Dolphins-Patriots game, things didn't come easily Sunday at Arrowhead Stadium.

The Chiefs appeared to be sleepwalking in the first half despite the fact that they were holding a 10-7 lead. Needing a big play to wake up, rookie wide receiver Mecole Hardman provided it in the second half.

The Chargers took a brief 14-10 advantage to start the third quarter when running back Melvin Gordon scored on a 5-yard plunge, which came three plays after quarterback a Patrick Mahomes interception.

Hardman produced the game-changing play on the ensuing kickoff with a 104-yard return down the left sideline. Hardman pulled a tight-rope routine near the Chargers' 35-yard line, maintaining his balance before cutting back against the grain of L.A.'s pursuit en route to the end zone.

The play provided the necessary energy to kick-start the Chiefs to the win.

"A spark, man," rookie Chiefs running back Darwin Thompson said. "It lit that fire and it just got crazy after that. We know they came to play with us, so we came with it."

"Sometimes that's what it takes," veteran Chiefs receiver Sammy Watkins said. "Guys like him to go out there and make a big play for us to wake everybody up."

The game turned in the Chiefs' favor after Hardman's play, and running back Damien Williams punctuated the awakening with an explosive 84-yard touchdown down the left sideline.

The Chargers appeared to have Williams stuffed for a short gain at the Chiefs' 18-yard line, but Williams bounced off two defenders, then spun free to give the home team a 24-14 lead.

Williams, who returned to the lineup in Week 16 after missing three games with a rib injury, totaled 124 yards rushing and two touchdowns on 12 carries against the Chargers. Over the past two games since his return, Williams has totaled 246 yards (189 rushing) and three touchdowns.

"I'm proud of him because he was injured during the year," Reid said. "He came back and played great."

Mahomes wasn't spectacular, but he didn't need to be as Hardman and Williams helped take the load off the quarterback's shoulders. Mahomes completed 16 of 25 passes for 174 yards and a touchdown, adding 21 yards on seven carries. And he came up big with a 47-yard completion to Tyreek Hill late in the game to help seal the win. Hill finished the game with four catches for 61 yards on five targets.

Meanwhile, Kansas City's defense allowed its most points since a 35-32 loss at Tennessee, snapping an impressive streak of five consecutive games in which they'd held opponents to 17 or fewer points.

The Chiefs' defensive unit seemed to take a bend-but-don't-break approach against the dangerous Chargers offense.

Chargers quarterback Philip Rivers threw for 281 yards and two touchdowns, but the Chiefs picked him off twice and effectively limited L.A.'s big plays. The Chiefs also kept running back Austin Ekeler in check, holding him to nine catches for 43 yards — a stark contrast to these teams' first meeting of the season in Week 11, when Ekeler caught eight passes for 108 yards on 12 targets.

Linebacker Anthony Hitchens paced the defensive effort with 10 tackles while safety Tyrann Mathieu added seven along with an interception and two passes defensed. Defensive tackle Chris Jones, defensive end Frank Clark and defensive end Terrell Suggs each recorded a sack; the Chiefs totaled eight hits on Rivers.

Suggs' sack gave him 139 for his career and moved him into eighth place on the NFL's all-time sack leaders list, ahead of DeMarcus Ware (138.5).

The defense also suffered a blow Sunday. Rookie safety Juan Thornhill, who has enjoyed a strong season alongside Mathieu in the back end of KC's coverage, suffered a left knee injury in the first half and left the sideline area on a cart before being ruled out for the rest of the day.

Armani Watts filled in for Thornhill and recorded three tackles. Reid said the team will know more Monday about Thornhill's status.

"We're hoping he's OK," Reid said.

Sunday's win gave the Chiefs a six-game winning streak, and a sweep of the AFC West for the first time since the 2016 season and the third time in franchise history. The Chiefs also finished the season with a 12-4 record, marking a second straight 12-wins-or-better campaign and the first time in team history the Chiefs have recorded consecutive 12-win seasons.

Reid is now 27-3 against the Chiefs' divisional opponents since 2015. Sunday's win also cemented for the Chiefs the NFL's best divisional winning percentage (.900) in a five-season span since the AFL-NFL merger in 1970.

With the regular season now over, the Chiefs will enjoy a first-round bye before hosting a game at Arrowhead in the AFC Divisional Round against the best remaining seed from the AFC Wild Card Round.