SANTA CLARA, Calif. (AP) — Brock Purdy and the San Francisco 49ers quickly turned the biggest challenge of their young season into a laugher.

Purdy threw three of his four touchdown passes to George Kittle, San Francisco’s defense hounded Dak Prescott all night and the 49ers extended their dominance over the Dallas Cowboys with a 42-10 victory Sunday.

“It felt good, especially going against the No. 1 defense in the league,” star left tackle Trent Williams said. “Obviously, that’s a big test for Week 5 of the year. I think we pass that test with flying colors. But we’re going to get tested again.”

The Niners (5-0) knocked the Cowboys out of the playoffs in tight games the past two seasons, adding intrigue to this early season matchup between perceived contenders.

The rematch wasn’t even close.

The Niners led 14-0 before the Cowboys got their initial first down of the night and were never really threatened on the way to matching a franchise record with their 15th straight regular-season win and earning their fifth 5-0 start.

“I knew it was our biggest game so far this year as far as nationally and just media wise and the hype,” coach Kyle Shanahan said. “Sometimes you worry your team can get too up for stuff. ... They were so locked in all week. They didn’t make too big a deal of this game and were ready to go.”

Purdy calmly picked apart the league’s stingiest defense to improve to 10-0 as a starter in the NFL and San Francisco’s defense made life difficult on Prescott all night.

Prescott was sacked three times and heaved an interception into double coverage after Dallas fell behind 28-10 in the third quarter. He threw two more interceptions in the fourth quarter with his only highlight coming on a 26-yard TD pass to KaVontae Turpin in the second quarter.

“We see the talent in the room and what our potential is, but it’s about doing it week in and week out,” said linebacker Fred Warner, who had a sack, an interception and a forced fumble. “To do it this game on this stage against a really good football team, it really meant a lot.”

It was Dallas’ most lopsided defeat since losing 49-17 to New Orleans in 2013.

“Didn’t see it coming,” Prescott said. “Put everything into this, and got punched in the mouth. It was humbling a couple weeks ago against Arizona. But this may be the most humbling game I’ve ever been a part of.”

Purdy again was nearly mistake free. A week after throwing only one incomplete pass in a win over Arizona, he repeatedly found openings downfield to Brandon Aiyuk, Deebo Samuel and especially Kittle.

Purdy finished 17 for 24 for 252 yards and has not thrown an interception all season. He connected on a 19-yard TD pass to Kittle on the opening drive, a 38-yarder early in the second quarter on a reverse flea-flicker and then a 10-yarder to put the game away in the third quarter.

“The defense does their job, so it makes life easy for me,” Purdy said.

Kittle became the first Niners player with three TD catches in a game since Vernon Davis in 2009.

Purdy added a 1-yard TD pass to Kyle Juszczyk in the fourth quarter and the rest of the game was merely garbage time with mostly backups.

“You have to learn how to take a punch,” Cowboys edge rusher Micah Parsons said. “We got punched in the face tonight and that’s just the reality of it. I’m not shying from it. We got beat up in all phases. And that’s the reality.”

FAST STARTERS

The Niners took the opening kick and drove 75 yards in seven plays to score on a 19-yard pass from Purdy to Kittle to take the lead. San Francisco has four TDs and one field goal on the opening drive this season for a league-leading 31 points.

The 49ers were just as tough on defense early, holding Dallas to 6 yards in the first quarter and no first downs for more than 17 minutes.

SCORING STREAK

Christian McCaffrey scored on a 1-yard run in the second quarter after losing a fumble near the goal line earlier in the game. It was the 14th straight game he has scored a TD in the regular season or playoffs. The only players in NFL history with longer streaks are Lenny Moore (17 games), O.J. Simpson (15) and John Riggins (15).

GET TRICKY

Shanahan said the Niners had the reverse flea-flicker in the game plan for a while, but had only used it once in the NFC title game last year when McCaffrey was the quarterback on the play. They did a walkthrough of the play this week that started with a handoff to McCaffrey, who handed to Samuel on a reverse before the flea-flicker to Purdy.

Shanahan got encouragement from watching Detroit score on a version of the play in the afternoon.

“We planned on calling it, but seeing it work for someone else helped,” he said.

INJURIES/INACTIVES

Cowboys: LB Leighton Vander Esch (neck) left in the fourth quarter. ... CB C.J. Goodwin (shoulder) and Turpin (ankle) left in the first half and didn’t return.

49ers: LG Aaron Banks (biceps) left in the second half.

UP NEXT

Cowboys: Visit Los Angeles Chargers on Oct. 16.

49ers: Visit Cleveland on Sunday.

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AP NFL: https://apnews.com/hub/NFL