Real Madrid cranked up the pressure again by edging to a win over struggling Real Valladolid, Casemiro's second-half header cutting the gap to just three points, albeit with Atletico still owning a game in hand.
"It's not because Atletico lost. We believe we can win the league," said Casemiro.
"There are still 42 points left, that's a lot of points."
The swing in momentum comes at a bad time for Atletico, who face Chelsea in the Champions League on Tuesday, and it feels significant given it has coincided with a resurgence, in La Liga at least, for their chasing rivals.
Real Madrid's victory at the Jose Zorilla Stadium was their fourth in a row, three of those coming with clean sheets, which suggests a return to form for a defence that all but clinched them the title last season.
Zinedine Zidane's side have done it without several key players too. Karim Benzema is the latest absentee through injury, joining the likes of Eden Hazard, Sergio Ramos and Dani Carvajal on the sidelines.
Benzema is struggling to be fit in time for Real Madrid's Champions League game away at Atalanta on Wednesday.
"It is better, we will see tomorrow, but we are not going to risk anything," said Zidane.
It meant a rare start for Mariano Diaz up front against Valladolid, while Isco was the only senior player on the bench.
Barcelona, meanwhile, play at home to Cadiz on Sunday when Ronald Koeman's side will be hoping to ease the pain of the Champions League thrashing by Paris Saint-Germain in midweek with an eighth consecutive win in La Liga.
For an hour, it looked like Real Madrid might suffer frustration against a determined Valladolid, who would have led had it not been for a string of excellent saves by Thibaut Courtois.
Courtois had to pull off a double stop to deny Fabian Orellana and Saidy Janko in the first half and then in the second blocked Orellana, who had been bizarrely set up by Ferland Mendy launching a ball across his own penalty area.
The Belgian kept Madrid in it and then Casemiro put them in front. Casemiro had been allowed two free headers already by Valladolid's defence and he pounced at the third opportunity, nodding in Toni Kroos' free-kick to give Real Madrid hope.
(With inputs from AFP)