The six-part series, starring Maxine Peake and Rupert Penry-Jones, continued with 5.08m (20.8%) on BBC One in the 9pm hour.
Silk easily outperformed Leah's Dream, a documentary about a child suffering with a rare form of dementia, which had 2.01m (8.2%) on ITV1 and 183k (1%) on +1.
The drama was also too strong for Heston's Mission Impossible, after the show cooked up 1.81m (7.4%) on Channel 4, down 310k week-on-week. A further 194k (1.1%) watched the programme on timeshift.
Earlier, The One Show entertained 4.74m (21.8%) on BBC One from 7pm, while Grimefighters had 2.71m (11.7%) on ITV1 from 7.30pm and 180k (0.7%) an hour later.
BBC One's Holby City fetched 5.91m (25%) in the 8pm hour, beating Lion Country's 2.19m (9.2%) on ITV1 and 69k (0.3%) on timeshift.
Over at BBC Two, Natural World captivated 1.67m (6.5%) in the 7pm hour, before A Farmer's Life For Me entertained 1.79m (7.6%) from 8pm. Horizon: Are We Still Evolving? educated 1.85m (7.5%) in the 9pm hour and Charlie Brooker's How TV Ruined Your Life amused 1.22m (6.2%) from 10pm.
Channel 4's Coach Trip rumbled on with 1.67m (10.3%) from 5.30pm and 266k (1.3%) on +1. Later, Gok's Clothes Roadshow had 2m (8.4%) in the 8pm hour and 194k (0.8%) an hour later. Shameless rounded out the evening with 1.65m (9.8%) from 10pm and 119k (1.3%) on timeshift.
Extraordinary Dogs mustered 572k (2.5%) on Channel 5 from 7.30pm, before documentary Stanstead: The Inside Story fetched 1.05m (4.4%). CSI and CSI: Miami brought in 2.19m (9%) and 1.26m (7%) respectively from 9pm.
Overall, BBC One dominated primetime with 24.3% against ITV1's 12% (+1: 0.5%). Channel 4 and BBC Two came in joint third with 7.2% (C4+1: 0.9%), followed by Channel 5 with 5.5%.
Elsewhere, Junior Doctors: Your Life In Their Hands mustered 1.15m (4.7%) on BBC Three in the 9pm hour, by far the biggest multichannel audience of the night.
BARB ratings data supplied by Attentional