Not since Real and Barca finished tied on 76 points in 2006-07, with Real crowned champions by virtue of a superior head-to-head record, and Sevilla stayed in the hunt down to the wire has there been such a close three-way fight for the title.
Barca’s 4-3 victory at the Bernabeu means Real and Atletico both have 70 points with nine games left, with champions Barca, chasing a fifth La Liga crown in six years, a point behind.
Atletico’s impressive achievements under Argentine coach Diego Simeone have shaken up a league increasingly dominated by Barca and Real, the world’s richest clubs by income.
Atletico have about a fifth of their wealthier rivals’ resources but the inspirational Simeone has managed to mould his unfancied squad into genuine contenders in Spain and on the European stage.
If they finish level on points with Real, Atletico will win the title, their first since 1996 when Simeone was a player there, as they have a better head-to-head record thanks to this month’s 2-2 draw at the Calderon and a 1-0 win at the Bernabeu in September.
Likewise, Barca have a superior head-to-head over Real after beating them 2-1 at the Nou Camp in October, the last time the Madrid club lost before Sunday.
Atletico and Barca drew 0-0 at the Calderon in Madrid in January and play each other at the Nou Camp on the final day in what could turn out to be a nail-biting climax to the campaign.
In the next round of matches, Real face a potentially tricky game at Sevilla on Wednesday (2100 GMT), when Barca host Celta Vigo (1900) and Atletico are at home to Granada (2100).
“The league is starting again from scratch and we are back in the battle for the title,” Barca coach Gerardo Martino told a news conference after his side’s success at Real, the Madrid club’s first defeat in 32 matches in all competitions.
“I always thought the three teams would be up there until the end and that’s how it has turned out,” added the Argentine.
“Today was our last chance and we did not want to be knocked out of the race.”
A win for Real on Sunday would have restored their three-point lead over Atletico and all-but killed off Barca’s challenge but instead they are licking their wounds after suffering only their second home defeat this term.
Captain Sergio Ramos was sent off in the 63rd minute for tripping Neymar when he was through on goal and the centre back and his team mates criticised referee Alberto Undiano Mallenco, suggesting he had not been entirely objective.
However, coach Carlo Ancelotti, like Martino in his first season in Spain, refused to be drawn into the debate and urged his players to focus on regrouping for the Sevilla game.
“Now we need to forget this match and think about the one on Wednesday, which could be a key match for the season,” the Italian told a news conference.
“Luck was not with us but the team showed good physical condition and we are hopeful and capable of quickly putting this game behind us.
“I have said many times that this league is open until the final day. We are not happy because we missed a good opportunity but not to worry.
“There are nine games left and it will be very intense for everyone. The league is open for the three teams and each match could be decisive.”
(Editing by Peter Rutherford)