PITTSBURGH — The boos tumbled from the mouths of nearly 64,000 black-and-gold clad fans sweating in the hot September sun at Heinz Field.

After watching their team claw back from a 21-0 first-quarter deficit, the Steelers fans watched in agony as the Chiefs' boy wonder quarterback showed off his arsenal of offensive weapons in the fourth quarter.

Quarterback Patrick Mahomes, a day away from turning 23, connected with Tyreek Hill for his sixth touchdown strike of the day.

Not only did the score give the Chiefs a two-possession lead en route to the 42-37 win, but it also tied Len Dawson's 1964 franchise record for touchdowns in a game. It was also the most touchdowns given up by the Steelers since a 1991 meeting against Buffalo.

The score also broke the NFL record set by Peyton Manning, Drew Brees and Charles Johnson for most touchdown passes in the first two weeks of the season with 10.

Unlike the last two low-scoring affairs between the two teams, Sunday's meeting deviated from the previous scripts _ and it's in large thanks to Mahomes.

Mahomes finished the day with a sterling 154.8 passer rating, completing 23 of 28 passes for 326 yards and six touchdowns.

With an abysmal defense that gave up 21 points in the second quarter and 485 yards overall, Mahomes had to be that good.

He opened up the first quarter by spreading the ball around, throwing three touchdowns to three different weapons: Chris Conley, Travis Kelce and Kareem Hunt.

The defense forced back-to-back three-and-outs and a missed 49-yard field goal, and the home crowd responded with jeers and boos.

And then in the waning minutes of a quarter where the Chiefs looked unstoppable and the Steelers looked pedestrian, it all fell apart.

Catalyzed by Orlando Scandrick's holding call on third-and-long late in the first quarter that wiped out a sack and defensive touchdown, the Chiefs' good fortunes stagnated.

Instead of getting quarterback Ben Roethlisberger (39 of 60, 452 yards, three TDs) off the field, the Chiefs (2-0) kept the Steelers' march alive. It resulted in Pittsburgh's first touchdown of the day. By the time the second quarter finished, the Steelers (0-1-1) completely wiped out the Chiefs' massive advantage and tied the game at 21 by halftime. Aided by the Chiefs' poor tackling and knack for racking up penalties, the Steelers dominated the second quarter with three long touchdown drives. The secondary struggled to make open field tackles.

Mahomes went to work quickly in the third quarter, hitting Kelce for the pair's second touchdown of the day on a 25-yard strike just two minutes into the period.

The defense tried to shore up its side of the ball, but it still allowed the Steelers to respond with a 5:30 drive. Frustration mounted when Steven Nelson intercepted Roethlisberger in the end zone during that drive, but the officials waved it off with a pass interference call. Upon hearing the officials' ruling, Andy Reid threw his headset off in disgust. Two plays later, the Steelers evened the score yet again thanks to a one-yard run by James Conner.

That's when Mahomes went to work yet again, absolving the defense's shortcomings with a four minute drive capped by a touchdown pass to Demarcus Robinson.

The strike to Hill gave the Chiefs a little breathing room, but not for long.

After tackling Kareem Hunt in the end zone for a safety, the Steelers carved up the Chiefs' defense yet again on a three minute drive that ended with Roethlisberger diving head first into the corner of the end zone.

Just like that, the boos of Heinz Field transformed to cheers as the Steelers' Styx anthem blasted through the stadium.

But the Chiefs offense salted away the final minutes of the game, and just as quickly, the cheers turned back into boos from the fans as they flooded the exits.