Nobody was better than the Vikings on third-down defense. Can they repeat in 2018?

If history will attach a superlative to the 2017 Vikings defense — the lifeblood of a 13-3 team, and a unit that allowed the ninth-fewest points in the NFL during this decade — it will likely be what the group did on third downs.

The Vikings allowed opponents to convert just 25.2 percent of their third-down attempts a year ago, setting a league record for the lowest third-down percentage since the NFL started tracking the statistic in 1991. It helped the Vikings allow only 252 points for the year, avoid giving up a single touchdown in situations where they were tied or ahead with four minutes or less to go, and become the first defense in Mike Zimmer's 18 years as a coordinator or head coach to finish atop the NFL in both points and yards allowed.

As the Vikings head into 2018, can they be that good on third downs again?

History would suggest they'll see some level of regression this year, as did the other five teams in this century who held opponents under 30 percent on third downs. None of the previous five — the 2003 Titans, Ravens and Broncos, the 2006 Ravens and the 2015 Texans — was better than 31.1 percent the next year, and four of the five saw at least a 5 percent drop in their third-down rate. There's a certain amount of randomness in any number of NFL metrics, and little breaks in a game — a dropped pass here, a friendly spot there — can affect some level of change in a team's performance.

Even if the Vikings aren't getting off the field at a historic level this season, they have plenty of reason to think they'll be among the league's best at it again in 2018.

Adding Sheldon Richardson to an already formidable front should help the Vikings generate even more of a pass rush, while helping to provide some sustainability in one of the things the team did best last year: generating stops on third-and-short.

The Vikings stopped 25 of their opponents' 64 third-down attempts from 1 to 4 yards, and were even better from 4 to 6 yards, stopping 33 of 44 attempts.

"I think then if you look at the statistics of the teams that are really good on third-and-1 — which last year I felt like we were pretty good — being able to defend the percentage from 2 to 4 and 5 to 7 and things like that go up quite a bit," Zimmer said. "You have to be really good at 6 and lower categories typically, and hopefully you're decent in the longer yardage ones. We spent a lot of time at it. We have some good cover guys; we have some good rushers. I think that all plays a part of it."

Third-down success certainly doesn't provide a complete picture of a great defense, and even if the Vikings do see their third-down rate slip to some degree this year, it doesn't mean they can't be just as good. The 2004 Ravens, for example, gave up 13 fewer points than their 2003 counterparts, even though they slipped to eighth in the league with a 34.4 third-down percentage.

The Vikings, though, have continued to emphasize the facet of their defense that became a calling card last year. And when the 49ers are in a third-down situation with fans at U.S. Bank Stadium at full volume on Sunday afternoon, they still figure to have a tough task in front of them.

"I don't think they're overrated, unless you're just giving up touchdowns on first down or something," safety Harrison Smith said. "Normally, you're good on third down if you're good on first and second — if you get them into second-and-long, or you don't give up a bunch of yards on first down so it's second-and-4 or something like that. It normally sets up success on third down."