Chelsea playmaker Deco believes February will be the pivotal month for the Blues' Premier League and UEFA Champions League aspirations.
Carlo Ancelotti's side endured a top-flight wobble in December, only taking nine points from a possible 18.
But the Blues are still at the top of the table, and Deco believes Ancelotti's are in pole position to lift the title, although February will be the key.
He told The Sun: "We have a good chance to win the title.
"We are first at the moment and we want to be in that position at the end.
"Who are our biggest threats? Probably Manchester United and Arsenal.
"Others like Manchester City and Aston Villa are close. City have some good players and they have brought in a new coach.
"But when you talk about our biggest rivals for the title you can only say Manchester United and Arsenal.
"We started well, but last month we didn't play so well.
"We are still in a good position. Our aim at the start of the season was to be top at New Year and we have achieved that.
"I think we can kick on - but February will decide everything. We play Arsenal and Manchester City in the Premier League and the Champions League gets back under way."
Deco has won the Champions League twice before, with Porto in 2004 and Barcelona in 2006, and believes it is harder to win than the Premier League.
He added: "Chelsea are one of the clubs who can win the Champions League, like Barcelona, like Real Madrid, like Arsenal and Man Utd.
"But it's more difficult to win than the Premier League. In the Premier League the best team will win.
"In the Champions League you can be the best team and still not win it. You might play well, the best, but there is always a chance you concede a goal or have some bad luck."