Ernesto Valverde believes the effects of a long season are being felt by Athletic Bilbao ahead of Thursday's Europa League quarter-final second leg with La Liga rivals Sevilla.
Holders Sevilla - who are seeking a third successive Europa League title - are in pole position to progress to the last four having claimed a 2-1 win at San Mames last week.
Europa League top scorer Aritz Aduriz put Bilbao ahead, only for Timothee Kolodziejczak and Vicente Iborra to give Sevilla the advantage going into the return fixture at the Ramon Sanchez Pizjuan.
Bilbao now face a tough challenge to turn the tie around at a stadium where Sevilla have lost just one of their last 18 matches in all competitions.
And, with forward Inaki Williams - who scored the only goal of the game in Sunday's 1-0 win over Rayo Vallecano - suffering a hamstring injury and joining Aymeric Laporte (ankle/leg) and Sabin Merino (hamstring) on the sidelines, Valverde accepts his team have a mountain to climb.
"There are so many demanding games. We're at a time in the season when the games pile up and the burden of the entire season is felt," Valverde said after the victory over Vallecano.
"After the result [in Bilbao], our chances [of reaching the semis] have dropped by 50 per cent, so we must exploit [the second leg] to the full.
"We're playing for the semi-finals and we have to go there with the idea of turning the tie around."
Sevilla will be without the services of the suspended Ever Banega and left-back Benoit Tremoulinas, who missed Sunday's defeat to Valencia because of a hamstring injury, is a doubt to feature.
Daniel Carrico has not played since March 17 but could make his comeback from a hamstring problem for the encounter.
Carrico's prospective return would be a welcome boost to a defence that was criticised by coach Unai Emery after their last-gasp 2-1 loss to Valencia.
At home in Europe, however, Sevilla have been tight at the back, going their last three games at the Ramon Sanchez Pizjuan in continental competition without conceding a goal.
Sevilla are also on a run of 12 consecutive home victories in the Europa League and, without Williams, there will be more emphasis on Aduriz to turn the tie on its head for Bilbao as they seek to end the hosts' recent dominance of the continent's second-tier club competition.
Key Opta Stats:
- Sevilla (4) and Athletic (2) have progressed in each of the previous UEFA Cup/Europa League quarter-final ties that they have been involved in.
- Bilbao have won two of their previous three European ties against Spanish teams; in the UEFA Cup against Barcelona in 1977 (QF) and versus Valencia in the Europa League Last 16 in 2016 (they lost the 2012 Europa League final vs Atletico Madrid).
- If Sevilla progress they will become the Spanish side to have beaten the most Spanish teams in knockout rounds and finals (7): Barcelona, Osasuna, Espanyol, Betis, Valencia, Villareal, Bilbao.
- Only Santi Mina (three goals, three assists) has had a hand in more goals than Sevilla's Kevin Gameiro (three goals, one assist) in the knockout phases of the Europa League 2015-16.