Girona are level on points with La Liga leaders Real Madrid after 19 rounds of fixtures this season.
Spain's biggest overachievers started 2024 by beating Atletico Madrid 4-3 on Wednesday.
Girona had never previously won a league game against Atletico.
But goals from Valery Fernandez, Savio and Daley Blind saw the home side move into a 3-1 lead inside 39 minutes.
Atletico were level by the 54th minute courtesy of a hat-trick from Alvaro Morata, who would have scored four goals were it not for a VAR intervention.
Morata still ended up on the losing team though because Ivan Martin struck a superb winner in added time to keep Girona's unlikely title bid on track.
Highlights: Girona 4-3 Atletico Madrid
Third-placed Atletico now sit 10 points behind Girona, who have never finished higher than 10th in Spain's top division.
Girona have won 15 of their first 19 league matches of the season.
Their only defeat was a 3-0 home loss against Real Madrid in September.
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