The Briton will face Frenchman Jo-Wilfried Tsonga, who beat Croatia's Ivo Karlovic 5-7 7-5 7-6 (8-6).
Murray will usurp Novak Djokovic as number one with titles in Vienna and at next week's Paris Masters, if Djokovic does not reach the final in Paris.
The Scot, 29, has never been top of the world rankings.
"Now I just try to get ready for the final, use today as a bit of rest day, practise a little bit on the centre court and then see what happens tomorrow," said Murray, who played an exhibition tie-break against Jurgen Melzer after Ferrer announced his withdrawal.
Ferrer had spent two hours and 33 minutes beating Viktor Troicki on Friday, and said: "It was impossible. I am disappointed but I have to be positive and will be ready for next season."
Murray has won 13 of his 15 matches against Tsonga, including a last-16 victory in their most recent meeting at Wimbledon in July.
Tsonga came from a set and 4-2 down to beat Karlovic, saving a match point in the tie-break as he reached his first ATP final of the year.
World number two Andy Murray is through to the final of the Erste Bank Open in Vienna after Spain's David Ferrer pulled out with a leg injury.