NBC scored a monster victory in primetime last week behind the season’s first two NFL games, but it was slower going for the net’s “The Voice,” which returned Monday to unspectacular ratings.

The music competish, which blossomed into NBC’s top-rated series in the second half of last season, is running its first fall edition and got a jump on its Monday competition by bowing two weeks before the start of the TV season. But, based on early ratings returns, a chunk of its aud isn’t in a rush to see the spinning chairs again so soon.

It’s tough to make comparisons to the premieres of the first two seasons of “The Voice,” as the show has now bowed in three disparate times of the year (late April, early February following a Super Bowl push and now early September). But Monday’s opener is clearly lower than the first two editions.

Monday’s third-season preem averaged a 4.2 rating/11 share in adults 18-49 and 12.29 million viewers overall from 8 to 10 p.m. — certainly good by any primetime measure these days, but underwhelming considering it didn’t face either ABC’s “Dancing With the Stars” or CBS’ comedy block, both of which bow Sept. 24.

“The Voice” premiered in late April 2011 with a big 5.1/13 in adults 18-49 and then exploded with its second-season premiere in February (6.7/17), which benefited from airing one night after the net’s coverage of the Super Bowl (and its own special preview hour about 30 minutes after the big game).

The show remained scorching hot for the rest of February but tailed off toward the end of the season, with the combo of Daylight Saving time and competition from ABC’s “Dancing” contributing to its downturn.

Monday’s 4.2 rating in 18-49 is well below the 6.0 that “The Voice” averaged in its four February regular-slot airings earlier this year. It’s also a bit below the show’s same-night average for its final 10 weeks last season (4.3 from March 5 through May 7) as well as its finale on May 8, a Tuesday (4.4).

There had been some thought that NBC would keep “The Voice” fresh by making it an annual event like Fox’s “American Idol,” but it’s hard to blame the net for trying to strike often while the iron is hot — and it’s the rare Peacock commodity that can provide a potent lead-in for new shows.

Monday’s season premiere of the “Voice” led into the special off-night bow of Ryan Murphy family comedy “The New Normal,” which received solid sampling (2.5/7, 6.88 million viewers overall). From next Monday it will air before “Revolution.”

On Tuesday, it led into comedy “Go On.”

ESPN’s “Monday Night Football” was the only program to rate higher than “The Voice” on Monday (4.9/14 in 18-49, 10.45m for San Diego-Oakland and 4.7/12, 10.90m for Cincinnati-Baltimore), though the doubleheader was down more than 10% from last year.

NBC had an easy time of it last week, racking up the best averages for any non-Olympics week on any net since Grammy week (Feb. 6-12) on CBS.

In adults 18-49, NBC’s 3.7 rating/11 share was followed by Fox (2.3/7), Univision (1.4/4), CBS (1.2/3) and ABC (1.0/3). And in total viewers, the net’s average (10.33 million) was nearly as much as Fox and CBS combined.

The NFL opener between the Dallas Cowboys and defending Super Bowl champion New York Giants, airing on Wednesday this year because of the Democratic National Convention, earned a big 9.7/26 in 18-49 and 23.89 million viewers overall.

But even that paled in comparison to the “Sunday Night Football” premiere between Pittsburgh and Denver (11.0/28, 27.57m), which featured the return to action of superstar quarterback Peyton Manning on his new team, the Broncos.

The game, won 31-19 by Denver, became NBC’s most-watched Sunday regular-season game since it bowed in 2006 while drawing the best young-adult score for any single-network primetime NFL regular-season contest in 15 years.

The final night of the Democratic convention from North Carolina averaged a combined 35.72 million viewers on 13 networks Thursday — well above the 30.25 million for the final night of the Republican gathering one week earlier but below the 38.38 million for the Dems on their concluding night four years ago.