The unusually hot weather led to the sci-fi series dropping to 6.19 million viewers, or a 41.8% audience share, between 7pm and 7.45pm.
A BBC spokesperson pointed out that the show was still the most-watched programme of the day and continued to be "king of Saturday evening."
Earlier, documentary Doctor Who: The Ultimate Guide had pulled in 2.77 million (23%) while on BBC Three, spinoff Doctor Who Confidential was multichannel's top draw at 677,000 (6.8%).
Casualty was the day's number two with 5.92 million (34.9%), while Murder in Suburbia ranked as ITV1's best-performing show with an average of 3.9 million (22.4%).
Elsewhere, Big Brother dipped to 2.4 million (13.5%), finishing fourth place in the 9pm hour (more).
Ratings data supplied by ViewingFigures.com