The Chiefs are the NFL's last undefeated team after cracking down on the 49ers in their Super Bowl rematch

A rematch between the reigning conference champions ended like the Super Bowl between them eight months ago.

The Kansas City Chiefs defeated the San Francisco 49ers 28-18 on Sunday, with the Chiefs improving to 6-0, the last undefeated team in the NFL, while an injury-riddled 49ers team fell to 3-4.

Brock Purdy and his undermanned pass-catching group were overwhelmed by the Chiefs' blitz attack plan.

Each team got off to a chaotic start on a dangerous day for the passing game, with Purdy and quarterback Patrick Mahomes combining for zero passing touchdowns and five interceptions.

Mahomes rushed for a score and Purdy for two, including a 1-yard touchdown with 1:08 to play that might have sparked a final comeback chance if the two-point conversion attempt had worked.

Instead, San Francisco will reflect on this loss while also determining the severity of a professional player's recent injury.

Brandon Aiyuk's right knee collapsed on the impact of a tackle on a 15-yard catch in the final minute of the first half. He grimaced as he was carted off and was soon forced out alongside Deebo Samuel (illness), Christian McCaffrey (calf/injured reserve) and Jauan Jennings (hip) as they were unavailable.

After the game, 49ers head coach Kyle Shanahan said the team feared Aiyuk would suffer an ACL injury.

And while Purdy had bright spots with his cast of second- and third-string receivers, the Chiefs' blitz-happy scheme was too much to overcome.

The Chiefs defense intercepts Brock Purdy three times

After the 49ers started the game with a three-and-out, the San Francisco defense repelled a fake punt attempt and stopped the Chiefs one yard short of fourth-and-2. But the Niners' momentum was short-lived as Purdy dropped back moments later and edge rusher George Karlaftis closed in and caught the pass intended for George Kittle 11 yards ahead of field safety Justin Reid.

Four more total interceptions would follow.

Mahomes made his first appearance on the ensuing series when defensive tackle Kalia Davis tipped over and then picked off his pass on 2nd-and-15.

The 49ers capitalized on the turnover with their first points of the day, a 55-yard field goal, thanks in part to a pass interference flag that Reid drew.

But with the Niners failing to find the end zone in the first half, the Chiefs ran tricks and Xavier Worthy's end-arounds, Mahomes' defensive diagnostics and Mahomes' physics so unusual that Tom Brady wondered on the broadcast whether Bill Belichick at home “on the field” was “tearing my hair out” and watching Mahomes “break every quarterback rule I learned.”

No matter: Mahomes did enough to get the Chiefs into the red zone five times, with running back Kareem Hunt rushing for the Chiefs' first two touchdowns on each occasion.

Tight coverage limited Purdy's targets while unblocked blitzers overran his mechanics and neutralized third-down attempts after another.

The 49ers' bright spot in the first half was receiver Ricky Pearsall catching a 6-yard pass with 1:32 to play in the first half, less than two months after the rookie was shot in a suspected armed robbery. Pearsall made his regular-season debut this week.

Whether this directly juiced the 49ers or the timing was fortuitous, Purdy completed his next three passes in a row — a 41-yarder down the left sideline to Kittle, a short pass to Jordan Mason and then a 15-yarder to Aiyuk.

But Aiyuk's knee gave out on the play, and the grimacing receiver was taken away after a visit to the blue medical tent. The 49ers' field goal before halftime cut their deficit to one point. But they would never catch the Chiefs.

The 49ers opened the second half with a determination that could not guarantee their luck on the day. When Worthy, the Chiefs' rookie receiver in the first round, slipped on a corner route after Mahomes released his pass, 49ers defensive back Deommodore Lenoir took advantage of the interception. As the passing game continued to struggle, Purdy gained nine yards on third down and then the quarterback keeper on fourth-and-1.

That would have been San Francisco's first touchdown, a Purdy sneak with three tight ends and two pulling right guards with 10:29 left in the third.

“That was pretty sick,” Brady enthused about the play’s design.

An apparent miscommunication between Purdy and a flatter-than-the-throw Ronnie Bell gave the Chiefs their second interception. The Chiefs marched down the field with a mix of running and passing plays, with Mahomes scrambling up the left sideline on second and seventh drives, faking pump gaffes and focusing his eyes on perceived dangerous areas enough to somehow reach 33 yards.

The Niners would never catch up.