Doctor Who's Steven Moffat has said that Matt Smith is "heartbreakingly good" in his final episode.

Shooting recently began on the BBC sci-fi drama's 2013 Christmas special - Smith's final outing as the eleventh Doctor.

"It's hard to let go - [I was] watching him on set... and he's so heartbreakingly good," showrunner Moffat told EntertainmentWise.

"You get all excited about the new doctor and then you go back and you see Matt on his first day back on set, and you think, 'Oh my God, that's what we're losing'."

Moffat added that Smith is "at the height of his powers" in his final Doctor Who appearance.

"He's so funny, so clever [and] so inventive," the head writer and executive producer continued.

Peter Capaldi will replace Smith as Doctor Who's lead actor, with Moffat calling his new star a "genius level actor".

"[Peter's] very excited and thrilled," he said. "He's a brilliant actor and he wanted the part, so that was it."

Doctor Who returns to BBC One on Saturday, November 23 for its 50th anniversary special 'The Day of the Doctor' - Smith's penultimate episode with the show.

> Doctor Who: Matt Smith shoots scenes for final episode - pictures

Watch Matt Smith and Jenna Coleman talk the end of the 11th Doctor with Digital Spy below:

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