Yesterday's latest instalment averaged 9.66m (36.9%) for ITV1 in the 9pm hour, before adding 325k (1.8%) on ITV1 +1.
Episode three of the third series, which centred around the disastrous wedding day of Lady Edith and Sir Anthony, ranks as Downton's third most popular - behind only last year's penultimate week and finale.
On those two occasions, however, Downton Abbey inherited an audience of 12m from The X Factor - last night the ITV singing contest drew 9.46m (35.4%) for the judges' pick of the final 12 ahead of the live shows in the 8pm hour.
Downton Abbey was the day's most-watched broadcast, overtaking X Factor for the first time in its history - never previously has Simon Cowell's show not topped the Sunday ratings.
The X Factor, which achieved a series high in its slot between 7.45pm and 9pm, added 382k (1.4%) on ITV1 +1 while The Xtra Factor took 735k (2.8%) on ITV2 (+1: 111k/0.6%).
Over on BBC One, Fake or Fortune? (4.25m/18.1%), Countryfile (5.4m/19.9%) and Andrew Marr's History of the World (2.54m/9.7%) all suffered dips.
On the other terrestrial channels, Big Fat Quiz of the 00s managed 1.45m (6%) for Channel 4 (+1: 276k/1.3%), while Dragons' Den entertained 1.47m (5.6%) on BBC Two, and 863k (3.2%) caught Channel 5's screening of Spider-Man 2.
Europe's dramatic victory over the USA in the Ryder Cup gave Sky Sports a gigantic audience across the day, averaging 1.28m (6.8%) from 4pm until 12.30am. Between 10.45pm and midnight, it was the top-rated show in its slot with over 2m.
Overall, ITV1 thrashed BBC One in primetime with 29.6% (+1: 1.1%) versus 15.8%. Sky Sports 1 rose to third due to its Ryder Cup coverage, while Channel 4 edged into fourth with 5.4% (+1: 1.2%), just ahead of BBC Two's 5.1%, and well clear of Channel 5's 3.5% (+1: 0.2%).
Kirk Norcross's surprise return to The Only Way Is Essex was seen by 844k (4.2%) in ITV2's 10pm slot, adding 183k (1.7%)
