Title not yet in the bag - Courtois
Jose Mourinho's men are four points clear at the top and are unbeaten through 11 league games this campaign.
But Courtois said it was far from over, expecting it to be "very difficult" despite the struggles of Manchester City, Arsenal, Manchester United and Liverpool.
"We don't have the title in the bag," he said.
"We are working game by game. The Premier League is very difficult.
"We are trying to win with every game that is possible. But there will be a still a long way to win the title. We are working hard. Hopefully that will be enough."
Third-placed City are eight points behind Chelsea, while Arsenal (12), United (13) and Liverpool (15) are even further adrift.