According to the BBC write up, 'Viewers in their millions deserted Doctor Who as it returned to BBC One for a new series, according to overnight figures'.
It was roundly beaten by ITV's X Factor, which had 7.3 million viewers. But reviewers were enthusiastic about the Doctor's return, with the Radio Times giving it five stars.
"We're now in the ninth series in 11 years since Doctor Who's revival, and it shows no sign of fatigue. The first episode rattles along with barely a bum note," wrote Patrick Mulkern in his review.
The Mirror said Michelle Gomez as Missy "steals the episode as the demented Time Lady who kills for fun and prances around in the face of greater evil".
And Catherine Gee in The Telegraph described the opening episodes as "packed full of jaw-droppers, one whopper of a cliffhanger and nerdy nods that date back to the time of Tom Baker."
Doctor Who was the most watched BBC programme of the evening, and ratings will rise once figures for catch-up and iPlayer requests are consolidated.