Jan Oblak says Atletico Madrid's 2-1 Supercopa de Espana defeat to Athletic Bilbao was "difficult to explain" and warned they must solve their defensive frailties. 

The LaLiga champions looked set to face city rivals Real Madrid in the final at King Fahd Stadium on Sunday after Unai Simon's unfortunate own goal put them in front just after the hour-mark.

Yet the holders mounted a great fightback in Riyadh on Thursday, Yeray Alvarez rising to head in a corner from Iker Muniain after 77 minutes.

Teenage substitute Nico Williams won it for Athletic nine minutes from time, finishing clinically with his left foot from inside the penalty area after Diego Simeone's side failed to deal with another corner.

Jose Gimenez's red card for a dangerous challenge on Inigo Martinez added insult to injury for Atleti.

Atleti have been vulnerable from set-pieces all season and goalkeeper Oblak says they have to tighten up.

He told MoviStar: "It is difficult to explain. Two set-pieces and this year we have conceded a lot [from set-pieces].

"We wanted to reach the final, we wanted to win the Super Cup, but we didn't do enough. I wish Athletic the best in the world, they won and they deserve to be there."

He added of Atleti's issues with defending set-pieces: "It is the question we want to solve, a lot of goals conceded this year. Many due to our failures. 

"It sure is a difficult time. You have a game like today where you go 1-0 and they score two goals from set-pieces.

"It is difficult to explain, we have to find the answer to this question. If it is not going to be complicated until the end of the season."

Oblak says Atleti can have no complaints with their defeat and must show a positive response.

"We have not played a good game. The plan was not that, it was to press and have more of the ball. The whole team is disappointed," he said.

"It is a difficult moment, you have to get up, with your head up, improve a lot. Each defeat is very hard and this one more because it was to go to a final for a title that we wanted a lot.

"We can talk, but words are useless until we solve it on the field."