Atletico Madrid's 15-game winning run came to an end on Saturday when they were unexpectedly beaten 1-0 by lowly local rivals Leganes, a defeat that could see them lose the LaLiga lead if Real Madrid defeat Las Palmas on Sunday.
Diego Simeone's Atletico still top the standings on 44 points, one ahead of second-placed Real and six clear of Barcelona in third, both with a game in hand.
Atletico dominated the game with more than 60% of ball possession, three strikes off the woodwork and twice as many goal attempts as the home side but paid a heavy price for too many squandered chances, including a penalty kick missed by Antoine Griezmann in added time.
Leganes scored the winner from a corner in the 50th minute, when Dani Raba crossed from the left to defender Matija Nastasic, at the far post, who fired a towering header past goalkeeper Jan Oblak and into the back of the net.
