The Red Devils are currently seven points clear of Manchester City at the top of the Premier League, but the Scot said that his team must remained focused.
He takes his team to Craven Cottage to face Fulham in today's late kickoff.
"It's very tempting to think about being 10 points clear," Ferguson told reporters. "But the most important thing is to keep our feet on the ground and try and win our game at Fulham. Then we'll see where that takes us.
"There's a long way to go. Fourteen games. A lot can happen.
"Nobody's a certainty in this league - they never have been. We were 11 points clear of Arsenal one year and lost the league. We were 12 points behind Newcastle one year and won it. It's an unremitting league. If you make mistakes you get punished."
United lost out on league glory on goal difference last year and Ferguson is keen to avoid a similar situation, but he admits that his team do not score as often as they should.
"It could go to goal difference again and that's an issue," he added. "At the moment we're only two goals better off than Manchester City but that could be undone in one game. We've had opportunities to be better."
City face Liverpool at the Etihad Stadium tomorrow.