Tom Brady set a record for the winningest quarterback in Super Bowl history, but the New England Patriots’ amazing comeback and overtime win against the Atlanta Falcons may not be enough to set record ratings.

In the overnight ratings, which measures television viewership in the nation’s biggest markets, the game averaged a 48.8 rating and 72 share, just below last year’s 49 rating and 73 share, according to Nielsen. The rating represents the percentage of TV households tuned into the game while the share indicates the percentage of people watching television at that time. Overnight ratings aren’t always an accurate barometer for national ratings, which will be released later Monday.

But the early ratings data do indicate that viewership peaked between 10 p.m. and 10:30 p.m. as the Patriots clawed their way back to force the first overtime in Super Bowl history.

Last year’s game ended up averaging 111.9 million viewers for CBS, which was the third-largest audience for a Super Bowl. The record audience remains the more than 114 million people who watched the Patriots beat the Seattle Seahawks on NBC in another nail biter in 2015.

Regardless of whether Super Bowl LI ends up being only the fourth most-watched television event in the country of all time, it is still a big win for Fox and the advertisers who paid about $5 million for 30 seconds of commercial time during the game. If the Falcons had continued pummeling the Patriots in the second half after entering halftime up 21-3, the audience likely would have fallen dramatically.

The Patriots comeback in overtime to win 34-28 kept a majority of the audience tuned in. That also bodes well for the premiere of Fox’s “24: Legacy,” which aired following the end of the game.

Fox is a unit of 21st Century Fox which shares common ownership with Wall Street Journal parent News Corp.

Write to Joe Flint at joe.flint@wsj.com

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