The BBC One adaptation of JK Rowling's Strike won the ratings war as it went head-to-head with the second series opener of ITV's Victoria.
Both programmes aired at 9.05pm on Sunday in one of the most hotly-contested TV battles of the year so far.
Crime drama Strike, based on Rowling's Cormoran Strike novels written under her pseudonym Robert Galbraith, debuted with an average of 5.5 million viewers and a peak of 6.2 million.
Victoria, back for its second outing after a successful first series, was watched by an average of 3.9 million viewers.
The episode peaked at 4.2 million, bringing it in at two million fewer viewers than its rival.
The overall audience share for Strike was 28.6%, while Victoria's was 21%.
This data is based on overnight viewers, and does not take into account those watching on catch-up services.
The BBC's audience success comes after its period drama Poldark was last year frequently beaten in the ratings by direct competitor Victoria.
Strike, starring Tom Burke as quick-tempered private detective Cormoran and Holliday Grainger as his eager yet fiery assistant Robin Ellacott, has started with a three-part series based on the first of Rowling's novels, The Cuckoo's Calling.
The second episode will air on Monday evening at 9pm, and the third on Sunday at 9pm on BBC One.
Jenna Coleman returned as Queen Victoria in the ITV's royal drama, the series picking up where it left off last year, and continues at 9pm on Sunday on ITV.
Elsewhere, BBC One's documentary Diana, 7 Days - which explored the week following the late Diana, Princess of Wales' death 20 years ago - was viewed by an average of 5.6 million.
The programme, which aired directly before Strike from 7.30pm, peaked with 6.9 million viewers.