MIAMI - The question the San Francisco defensive backs will spend the off-season asking themselves was probably the same one going through the mind of Patrick Mahomes in the split second that changed the Super Bowl on Sunday.

How did Tyreek Hill get that open?

Clogged up for more than three quarters by Richard Sherman and a Niners defence that didn’t give up big plays this season, Mahomes and the Chiefs finally found their opening. It was a 44-yard pass to Hill, the small-and-speedy wide receiver who got behind not one, not two, but three defenders on third-and-15 and with KC’s hopes looking bleak in the fourth quarter.

That was the turning point in Kansas City’s 31-20 victory, the key to its 21-point fourth quarter and the trigger point in the Chiefs’ third straight comeback from a double-digit deficit in this year’s post-season. This final one, on a cool crisp night in South Florida, earned them the Lombardi Trophy.

“Coach Reid told me ... to keep firing, keep believing in your eyes and go for it,” Mahomes said.

Hard to imagine he could believe his eyes when he saw Hill so wide-open.

The 49ers came in having allowed only eight completions this season on passes thrown more than 20 yards downfield — the fewest of any team in the NFL in 14 seasons. For nearly 53 minutes in this one, they were living up to the resume.

Harassing Mahomes in the backfield, never letting him get free on the edges, let alone wind up for a big throw, the Niners were in total control.

They forced two Mahomes picks — the first two of his young post-season career — and celebrated each time by posing for fake photos in the end zone. They took a 20-10 lead that felt even bigger.

“I knew we weren’t in the ideal situation,” Mahomes said.

Trailing by 10 and with desperation time nearing, Mahomes threw a pair of incompletions, including one that was initially ruled a catch by Hill but was overturned on review. That made it third-and-15 at the 35; one more stop by the Niners would’ve pretty much brought an end to the game.

Mahomes took the snap and backpedaled 13 yards. He looked downfield, then to his left, and heaved his longest throw of the evening.

“I think the offensive line gave me enough time to throw a really deep route, and I just put it out there and Tyreek made a really great play,” Mahomes said.

Hill stopped in his tracks and cradled it in. Only then did three Niners — Emmanuel Moseley, K’Waun Williams and Fred Warner — swarm back to tackle him at the 21.

Given Hill’s nearly track-star speed, most of the focus this season has been on containing him on the edges and not letting him catch the ball and take off. But this was a different sort of long gain.

“No, no, no, no, no,” Hill said, when asked if he was surprised to be that open. “You’re never surprised if you’re speedy. I was just able to use my speed, get up on the d-backs, get him to roll over and get open.”

Four plays later, the Chiefs had cut the deficit to three.

The San Francisco defence was stunned and never overcame it.

There wasn’t much in the way of explanation in the Niners’ still-stunned postgame interview area.

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“I’m not sure exactly what happened,” coach Kyle Shanahan said. “It looked like we were playing zone defence. It looked like he was allowed to hold the ball a pretty long while. And Tyreek got behind us on a corner route.“

Warner called it simply a matter of Mahomes having “time to air it out and we didn’t have the guys back there to make the play.”

And DeForest Buckner, part of a line that kept Mahomes off-balance all night (he was sacked four times), said, “We knew we needed to make one more play, and we didn’t.”

“We didn’t get any pressure and we left them hanging out to dry back there. It’s the worst feeling in the world,“ Buckner said.

After cutting the deficit to 20-17, Kansas City forced a three-and-out. Four plays into its next drive, Sammy Watkins juked Sherman at the line of scrimmage and got behind him to set up an easy 38-yard reception that led to the go-ahead score.

Kansas City got the ball back again once more, and Damien Williams broke a 38-yard run for the last touchdown.

That made it 21 points in the fourth quarter — over the last 6:13 to be exact.

That made a 10-point deficit an 11-point win.

That brought the Super Bowl trophy back to Kansas City for the first time in 50 years.

“I wasn’t feeling good about it at all. I told Pat, ‘It’s 20-10 with seven minutes left — c’mon bro,’” Hill said. “And all Pat did, he just told me to believe.”

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