A peak of 9.8m viewers saw Francine Lewis and Asanda Jezile move into next week's grand final from 7.30pm, while a further 467k (2.1%) tuned in on +1.
The results show was seen by 7.2m (32.7%) at 9.30pm (391k/2.5% on +1), while Britain's Got More Talent entertained 702k (4%) at 10pm on ITV2.
Have I Got News for You and Not Going Out earned 3.81m (16.5%) and 3.32m (14.7m) respectively from 9pm on BBC One.
Would I Lie To You grabbed 2.63m (11.7%) at 8.30pm and The Graham Norton Show was watched by 3.6m (26.3%) at 10.45pm.
On BBC Two, The Time Traveller's Guide to Elizabethan England interested 1.77m (7.7%) from 9pm, prior to which Gardeners' World had 1.85m (8.3%) at 8.30pm.
Meanwhile, Channel 4's Sarah Millican: Thoroughly Modern Millican Live had 910k (4%) at 9pm and Alan Carr's Chatty Man was watched by 1.17m (6.8%) at 10pm.
Eddie Stobart: Trucks and Trailers took 700k (3.3%) at 8pm on Channel 5, followed by Death Race with 5.97k (3%).
Lewis was the highest rated show across the digital channels, earning 747k (3.4%) for ITV3 at 8pm.
Relive Britain's Got Talent's fourth semi-final in the gallery below:
BARB ratings data supplied by Attentional