TV Ratings: 'Sherlock's' U.S. Audience Grows After Long Break -- Still No 'Downton Abbey'

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In addition to welcoming Mary Morstam (Amanda Abbington) into the fold, Sherlock also will introduce Sherlock's nemesis Charles Augustus Magnussen (Lars Mikkelson).

Sherlock jumped 25 percent after a nearly two-year absence from PBS. The Masterpiece Mystery debut of the U.K. hit, airing on just a three-week delay, averaged 4 million viewers on Sunday night.

That's up 800,000 from the season-two launch in May 2012, easily marking a stateside series high for the show. The lapse between the second and third season has seen both stars, Benedict Cumberbatch and Martin Freeman, become increasingly familiar to U.S. audiences -- thanks to roles in blockbuster franchises Star Trek and The Hobbit, respectively.

Sherlock also had an enviable lead-in from Downton Abbey. Three weeks into its U.S. run, which is on a much bigger delay from the U.K., the drama averaged 8.4 million viewers. That's down quite modestly from the Jan. 5 record-setting premiere of 10.2 million viewers.

Downton Abbey remains a much bigger British import. The recent premiere was up 22 percent from a year ago. And the 2013 return marked an unprecedented 96 percent surge from 2012.