It's hard to win every week in the NFL. We're just five weeks into the season and only two undefeated teams remain.

The San Francisco 49ers are closing in on a full calendar year without losing a regular-season game.

The 49ers are the best team in the NFL through five games and it's not all that close. The argument against them before Sunday night might have been that they hadn't played anyone. Then they took on the Dallas Cowboys, a legitimate Super Bowl contender, and destroyed them. Behind Brock Purdy's four touchdown passes, the 49ers controlled every part of the game in a 42-10 win over the Cowboys. The 49ers and Philadelphia Eagles are the NFL's only two 5-0 teams.

The last time the 49ers lost a regular season game was Oct. 23 last year. They lost to the Kansas City Chiefs a few days after acquiring Christian McCaffrey in a trade. If the 49ers win their next game, at the Cleveland Browns, they'll make a full calendar year between regular-season losses. Their Week 7 game is on Monday night, Oct. 23.

The way the 49ers looked against the Cowboys, it'll be tough for anyone to break that streak.

49ers dominate against Cowboys

A big part of the 49ers' winning streak has been Purdy. The 49ers have yet to lose a regular-season game that Purdy has started. Purdy still has a way to go to reach Ben Roethlisberger's record of 15 straight wins to start a career, but he's gotten to 10 in a row.

Of course, this all has to include the qualifier of "regular season." The 49ers lost in the NFC Championship game to the Eagles last season, a game that Purdy exited with an elbow injury. But if the 49ers keep playing like they have to start this season, they'll have a chance to make up for that postseason loss. There's no weakness, as they showed on Sunday night.

The 49ers started fast, as they have most of the season. Purdy hit George Kittle for a touchdown on their first possession. Later in the first half, the 49ers ran a reverse that was pitched back to Purdy, who hit an open Kittle downfield for another touchdown. It was the mirror image of a play the Detroit Lions ran earlier in the day, and it worked perfectly.

After the game, 49ers head coach Kyle Shanahan acknowledged he'd seen the Lions run it earlier in the day, though he said his own team has had it in the playbook for awhile.

After that touchdown, the 49ers led a very good Dallas team 14-0. It would have been 21-0 if not for a rare McCaffrey fumble near the goal line. McCaffrey made up for it later by scoring a touchdown in his 14th straight game, counting playoffs, extending his team record. That made the score 21-7.

The Cowboys are still one of the seven or eight best teams in football. Top 10 at worst. And they had no chance against the 49ers. That's scary.

49ers continue blowout after halftime

There was a telling sequence near the end of the first half. The Cowboys were down 21-7. In the NBC booth, Cris Collinsworth started going on about how Dak Prescott had a chance to drive his team down in the final minute for points and some momentum. Dallas threw incomplete, took a sack on second down and then, on third down with two timeouts left, just decided to let the rest of the half run out. They needed points desperately but were too worried about the 49ers' defense to even try another play. It's not like any halftime meetings were going to solve how to beat the 49ers, on offense or defense.

The second half didn't look much different. Purdy hit Kittle for the third touchdown connection between the two. Then Purdy hit fullback Kyle Juszczyk for his fourth TD of the night. San Francisco's defense continued to harass Prescott. That's why the Cowboys shouldn't have just let the first half run out. They needed every opportunity to score points to keep up with the 49ers. But also, it's understandable that Cowboys coach Mike McCarthy was aware that his team wasn't moving it anyway and just wanted to cut his losses.

The Eagles are undefeated too, but haven't been blowing the doors off of opponents. There's nothing wrong with Philadelphia's 5-0 record, but the 49ers have been unquestionably more dominant. Their offense has an endless line of playmakers and the defense is ferocious. The Eagles and 49ers play on Dec. 3 in Philadelphia. It's becoming a real question of whether the 49ers might still be undefeated by then.