After losing Eder Militao to a red card within the first 10 minutes, Real Madrid suffered a 2-1 humbling defeat against Levante on Saturday, handing Atletico Madrid the chance to move 10 points clear at the top of La Liga.
The defending champions sit second in the table, seven points behind Atletico, who have two games in hand and face Cadiz on Sunday.
Militao was shown a red card in the ninth minute at the Alfredo di Stefano Stadium, with his team already without their talismanic captain Sergio Ramos due to an injury and COVID-stricken coach Zinedine Zidane.
However, four minutes later, Marco Asensio gave Madrid the lead but Jose Luis Morales equalised before half-time and soon Roger Marti made up for missing a penalty by scoring the winner.
Barcelona coach Ronald Koeman had admitted earlier on Saturday his team "have to be realistic" about their chances of overturning Atletico's considerable lead in the table, AFP reported.
And with Real Madrid slipping up again, it will now take a seismic collapse for Atletico to miss out on repeating their incredible La Liga triumph in 2014.
It also leaves the Champions League as Real Madrid's only obvious chance of a trophy this season after they lost in the Spanish Super Cup semi-finals and went out to Alcoyano in the Copa del Rey. Madrid face Atalanta next month in the Champions League's last 16.
Militao was sent off for bringing down Levante's Sergio Leon, who was darting towards goal when Militao cut across him on the edge of the penalty area.
But Madrid gave themselves a lead to hang onto shortly after when Asensio continued his recent resurgence by sprinting onto a brilliant Toni Kroos through ball and finishing into the corner.
Levante were dangerous, though, and drew level before the interval as Jorge Miramon's cross to the back post was expertly caught on the half-volley by Morales.
Vinicius Junior came on for Madrid in the second half but made the worst possible start by kicking away the leg of Carlos Clerc and the VAR decided that the foul was inside the area and referee Medie Jimenez pointed to the spot.
Thibaut Courtois fisted away the penalty by Roger and Madrid was still seemingly in the race to seize the three points.
But instead, their tired legs gave way and a well-worked short corner by Levante gave a clear sight of goal to Roger, who made amends by firing into the corner.