Gennady Golovkin's second-round knockout of Vanes Martirosyan also was a power punch for HBO.
The middleweight title bout at StubHub Center generated an average of 1.249 million viewers, peaking with 1.361 million, to become the top-performing fight on premium cable this year, according to Nielsen Media Research.
Golovkin (38-0-1, 34 knockouts) pushed to take the bout after Canelo Alvarez was suspended for six months by the Nevada Athletic Commission for submitting two positive samples for the banned performance-enhancing substance clenbuterol.
It was Golovkin's record-tying 20th consecutive middleweight title fight without a loss, matching the record by Alvarez co-promoter Bernard Hopkins, and it came in a nine-punch flurry of power blows that stopped the challenger from Glendale.
HBO's ratings were its best for a non-pay-per-view fight since Golovkin knocked out Dominic Wade at the Forum in 2016.
And the audience exceeded even the top ESPN boxing telecast of the year, in which Oscar Valdez retained his featherweight title by defeating England's Scott Quigg at StubHub Center, which drew an average of 1.1 million viewers in March.