Vettel won the 2012 drivers’ championship by just three points – and Horner is expecting a similarly tight affair this year.
He said: “It would foolish to sit here and underestimate the calibre of our opponents – both drivers and teams – because last year it went all the way to the wire and in 2010 it went all the way to the wire.
“I’m sure this year is going to be another tough and tight season.”
Horner added: “Our focus is very much based on 2013 at the moment, but we have a small group of people looking towards 2014.
“It’s a matter of spinning the two plates and balancing the defence of our titles in 2013 while also looking towards 2014.
“It’s a matter of balancing your resource accordingly and that becomes harder as the season progresses.”