'The Great Game', written by Mark Gatiss, pulled in 7.01m (29.9%) on BBC One, alongside a further 325k on HD.
Benedict Cumberbatch's Sherlock Holmes has been popular with both viewers and critics, with the first three-part series averaging a strong 7.2m.
Meanwhile, The Unforgettable Bob Monkhouse was a long way off in second place in the 9pm hour, with 3.27m (13.5%) for ITV1. Coast put in 2.64m (10.9%) for BBC Two, and Big Brother mustered 1.99m (8.2%) for Channel 4.
Earlier in the evening, Last of the Summer Wine was seen by 3.91m (20.2%) on BBC One.Countryfile followed with 4.25m (18.8%), beaten by Heartbeat, which logged a solid 4.59m (20.3%) for ITV1. Prior to that, Countrywise at War interested 2.99m (16.2%).
BBC Two's Top Gear repeat grabbed 3.27m (14.5%) at the same time, way ahead of the 1.94m (8.6%) watching Channel 4's Amish: World's Squarest Teenagers. Over on Five, a screening of Vertical Limit entertained 565k (2.4%).
Later at 10pm, Britain Goes Camping pipped Alan Carr: Chatty Man, with 1.64m (8.6%) to 1.2m (6%).
Overall, BBC One eclipsed ITV1 in primetime, with 24.3% to 15.5%. BBC Two followed in third place with a respectable 11.1%, while Channel 4 and Five averaged 6.4% [+1: 0.8%] and 2.7% respectively.
BBC Three topped Five in primetime, with 3.2%. Lara Croft: Tomb Raider - The Cradle Of Life and Family Guy notched up 763k (3.7%) and 706k (3.4%).
BARB ratings data supplied by Attentional