HOUSTON -- Thanks to another sparkling performance from Houston Texans rookie quarterback Deshaun Watson, what once appeared to be an insurmountable Kansas City lead turned tenuous enough in the second half that Chiefs quarterback Alex Smith was required to do more of what he has done all season.

Smith tossed three touchdown passes and engineered a decisive scoring drive in the fourth quarter of the Chiefs' 42-34 win over the Texans on Sunday night at NRG Stadium.

Smith authored a response after Watson completed his second second-half scoring pass to receiver Will Fuller, a 48-yarder with 12 minutes left that cut what was a 16-point deficit to 26-20. Watson wound up throwing for five touchdowns.

Needing a reply, Smith led a six-play, 70-yard march that required just 3:03 to complete. His 10-yard touchdown pass to De'Anthony Thomas extended the lead to 32-20 with 8:57 left.

"Once they made it a one-score game, I knew the onus was back on us to go answer," said Smith, who finished 29 of 37 for 324 yards and did not throw an interception. "If you keep answering them, it kind of becomes demoralizing when you make it a two-score game again."

Receiver/return specialist Tyreek Hill ensured that the Chiefs (5-0) would remain as the lone unbeaten team in the NFL when he returned a punt 82 yards for a touchdown with 7:22 left.

The Texans (2-3) not only dropped the game, but they also lost three-time Defensive Player of the Year J.J. Watt to a tibial plateau fracture in his left leg during the first quarter.

According to multiple media reports, Watt will undergo season-ending surgery. He participated in just three games last season before undergoing back surgery that kept him out the rest of the year.

Watson, who last week threw for four touchdowns and ran for another in a win over the Tennessee Titans, completed 16 of 31 attempts for 261 yards Sunday. Four of his scoring passes came in the second half, keeping the Texans in contention. Receiver DeAndre Hopkins caught three of the touchdown passes.

"We played a good team," Hopkins said. "A team like that, you can't make many mistakes because they're a good team. They're undefeated for a reason, so got to give your hats off to the Chiefs."

The Chiefs amassed 450 yards and 29 first downs. Rookie running back Kareem Hunt, the focal point of the Texans' defense in the first half, still managed 107 rushing yards on 29 attempts.

Smith produced a breathtaking display for the Chiefs prior to the intermission, with Kansas City converting 7 of 10 third downs en route to scoring on all five of its first-half possessions.

With the Texans keying on Hunt, Smith passed for 207 yards and two touchdowns in the half.

Tight end Travis Kelce was the primary beneficiary, turning 11 targets into eight receptions for 98 yards. Kelce was evaluated for a concussion at halftime and did not return.

Smith completed both of his first-half scores to running back Charcandrick West, covering 8 yards for a 13-0 lead with 5:45 left in the second quarter before capping a seven-play, 75-yard march in just 75 seconds with a 4-yarder with 34 seconds left in the half.

"He's been the starter and handled that well," Chiefs head coach Andy Reid said of West. "Now, he's kind of the relief pitcher, and he's handling that really well, too."

The Chiefs' final scoring drive in the first half answered the Texans' lone sustained march before the break, one that concluded with Watson finding Hopkins for a 6-yard strike in the back of the end zone.

After possessing the ball for just 8:17 in the first half, Houston found its stride. Still, the Texans couldn't slow Smith and the red-hot Chiefs.

"He's really good at extending plays," Texans head coach Bill O'Brien said of Smith. "He got outside the pocket. That was one of the keys to the game: We had to try to keep him in the pocket, and obviously, we didn't accomplish that."

NOTES: Texans LB Whitney Mercilus was lost to injury on the first defensive series, and the Houston Chronicle reported he will undergo season-ending surgery to repair a torn pectoral muscle. Mercilus recorded 7.5 sacks last season. ... Chiefs RG Cam Erving, acquired from the Cleveland Browns at the close of training camp, got the start in place of Laurent Duvernay-Tardif (knee). Erving, a first-round pick of the Browns in 2015, started 17 games for Cleveland in two seasons, including 13 in 2016. ... Chiefs S Steven Terrell was knocked out of the game with a concussion on the opening kickoff.