Saul’s early strike handed Atletico Madrid a narrow 1-0 win over Liverpool in the Champions League knockout round first leg.
Liverpool fielded what was arguably their strongest line-up for the first time this season, with Fabinho slotting into the midfield three at the expense of Naby Keita and Sadio Mane returning from a hamstring injury.
But it was Atletico who started the brighter of the two sides and the signs of danger were there when Renan Lodi’s cross eluded Alvaro Morata by a matter of inches.
With only five minutes on the clock, the Spanish giants went ahead.
Virgil van Dijk gave away a corner with an unusually panicked clearance and Koke’s delivery caused mayhem in the box before striking Fabinho.
Saul reacted first and calmly stroked past Alisson to send the home fans into frenzy.
That goal only served to increase the decibels inside the already-deafening arena and put Liverpool behind for the first time since they drew 1-1 with Napoli.
The holders found themselves up against a resilient backline, camped on the edge of their own box in two banks of four.
But when Liverpool lost possession, Morata and Angel Correa looked ready to pounce.
And the former missed a huge opportunity to make it 2-0 when he capitalised on an error from van Dijk only to see his effort blocked by Alisson.
Liverpool were struggling to find a way past the Atletico blockade in front of Oblak and it proved again when Mohamed Salah, some 12 yards out, saw his goalbound effort hit Felipe rather than the back of the net.
Klopp took decisive action at the break, replacing Mane with Divock Origi who had proven his capability of grabbing a crucial goal in the past.
But their best chance fell to Salah when he was found by Joe Gomez with a deep cross only for the Egyptian to glance his header wide of the target.
Atletico continued to sit deep and frustrate the Merseyside outfit and with Salah failing to make an impact, Klopp sent on Alex Oxlade-Chamberlain.
Captain Jordan Henderson came the closest to levelling the score, volleying wide from Origi’s cross, and the Reds were dealt a big blow when the midfielder went off with an injury.
With Diego Simeone proving to be Atletico’s biggest cheerleader, encouraging more noise from the supporters, his side responded with some tireless work off the ball to nullify Liverpool’s front three.
Meanwhile, his opposite number Klopp threw his hands up in frustration after seeing the referee award fouls against his players and the home side – buoyed by their noisy supporters – held out for an impressive, hard-fought victory, leaving Liverpool with work to do in the second leg at Anfield.