At its peak, 9.8m people - 47% of the total TV audience - tuned in to see the Time Lord defeat the Daleks, according to BBC figures.
The average viewing figure of 9.4m is 300,000 more than watched the first episode of the series in April.
The f
igures for Doctor Who followed days of intense speculation over Tennant's future in the Tardis.
A BBC spokesman said: "We're absolutely delighted with the figures and viewers can now look forward to the Christmas Special."
However, Tennant's longer term future as the Time Lord remains uncertain.
He will star in a Doctor Who Christmas special this year and another three specials next year but is not confirmed for the next series in 2010.
The fifth series has been postponed so that the Casanova star can take to the stage to play Hamlet with the Royal Shakespeare Company.
Bookmakers, meanwhile, were forced to close their books on who will be the next Time Lord.
Bookies yesterday installed David Morrissey as favourite to take over, the 44-year-old actor going from a 33/1 outsider to an odds-on hot tip in just a few hours.
Betting giants Ladbrokes and William Hill closed their books, with Hill's spokesman Rupert Adams said: "Some callers were asking for bets of £5000. Gambles such as these are rarely wrong."
William Hill's final odds on who will take over the Tardis: David Morrissey, 1/6 favourite; Robert Carlyle, 3/1; James McAvoy and Jason Statham, 6/1; Alan Davies, John Simm and James Nesbitt, 8/1; Daniel Radcliffe, 12/1; a female Doctor, 25/1.
