Atletico Madrid were in dreamland as they destroyed rivals Real Madrid with a thumping 4-0 win which blew the Primera Division title race wide open.
The defending champions set the tone from the very start at Vicente Calderon Stadium as Tiago Cardoso and substitute Saul Niguez scored early before Antoine Griezmann and Mario Mandzukic sealed the lopsided victory in the second half.
The return of Ballon d'Or winner Cristiano Ronaldo didn't help an injury-hit Madrid, which also got poor showings from its other star players.
"Madrid can't lose to any team 4-0," Ronaldo said. "We were lacking everything, desire, attitude."
Diego Simeone's side extended their undefeated streak to six meetings with Madrid since losing last season's Champions League final in added time.
Atletico may have had more important victories over its fiercest rival since ending Madrid's 14-year reign of their contests in the 2013 Copa del Rey final, but they had yet to beat Madrid at home in the league since the 1998-99 season.
Simeone, however, downplayed the significance of Atletico's most complete victory over Madrid in years, and one their fans will relish for a long time. "It's just another league match," said Simeone, who also dusted off his mantra of "going forward game by game" when asked if the win rekindled his team's bid to repeat.
"We have our philosophy of going forward one game at a time, partly because we don't know any other way."
Atletico pulled to within four points of Madrid.
Madrid were without injured first-choice players such as James Rodriguez, Sergio Ramos, Pepe, and Luka Modric, while Marcelo was suspended.
But it still had enough star power with Ronaldo back from a two-match ban for kicking an opponent. Plus Gareth Bale, Karim Benzema, and midfield pair Toni Kroos and Francisco "Isco" Alarcon. None could stem the red-and-white tide.
"Atletico beat us in every facet. We didn't have a player who had a good game," Madrid coach Carlo Ancelotti said. "This is the worst game by my team since I have been coach."
Since finishing 2014 on a Spanish record 22-win streak, Madrid has been eliminated by Atletico from the Copa del Rey, and beaten a second time in the league this season.