At the end of every match and every training session for two years Saúl Ñíguez left the pitch, headed to the toilet and pissed blood. Every day it hurt, every day was a risk but the Atlético Madrid midfielder did not stop. Nor, though, did he entirely leave it behind until he returned to the place where his career could have ended – to the place his kidney burst.

That night in February 2015 at the BayArena in Leverkusen, barely able to stand, he had departed in the arms of the physio, vomiting repeatedly as they made their way to the dressing room; they had to stop seven times. Two years on he left the same arena having scored as Atlético beat Bayer Leverkusen 4-2 to put themselves on course for the Champions League quarter-finals where they met Leicester City. When they travelled to the King Power, having won the first leg 1-0, Saúl headed the goal that effectively ended the tie. Next up are Real Madrid, the club Saúl joined and left as a kid, the victim of bullying.

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This is the fourth time Atlético have faced Real in the Champions League in four years, two of those in the final, first Lisbon in 2014 and then Milan in 2016. The last time they met in a knockout round was in spring 2015 in the quarter-finals. But Saúl was absent because of the injury in the previous round that had changed everything except his determination to play. They missed him: aged 20, scorer of an overhead kick against Real three weeks earlier in La Liga, he was coming into superb form when a first-half clash with Bayer Leverkusen’s Kyriakos Papadopoulos left him, in the words of the club’s president Enrique Cerezo, with his “kidney destroyed”.

“I think it was my first start in the Champions League and that happens,” Saúl recalled. “It was the worst night of my career, not just because of the injury itself but seeing my dad there in tears.”

Saúl carried on but could take no more and was eventually withdrawn just before half-time. In the dressing room he started to shake and then lost the feeling in his arms and legs, so he was taken to hospital by ambulance, accompanied by his terrified father. His team‑mates flew back to Madrid after the game but Saúl, who had previously suffered a renal colic while on loan at Rayo Vallecano, stayed in hospital in Leverkusen for four more days as the blood was drained. He did not play for a month but soon returned.

And that, it seemed, was that. Silently he suffered. Publicly he kept on playing – and ever more impressively. He has made over 100 appearances since then, including a Champions League final – Real again, another agonising defeat – which Atlético had reached thanks to his wonderful goal against Bayern Munich in the first leg of the semi-final at the Vincente Calderón, running from deep through the opposition defence. Now, for a second successive season, they aspire to make it to another final, this time in Cardiff.

Facebook Twitter Pinterest Saúl Niguez celebrates scoring against Bayer Leverkusen. Photograph: Reuters

Saúl has been vital in them getting this far, well beyond the goal at Leicester. His role had changed, a shift more important still because of injuries to Thiago and Augusto Fernández and the difficulties accommodating Koke centrally.

At Atlético they always knew Saúl had talent; it is the tactical maturity that is striking. He has always been self‑assured – he admits talking too much when he was younger – and saw himself as an attacking No8, driving into the area, and until this season he had played to the right or the left. Now, though, he has become “another Gabi”, according to one former youth team coach: deep, defensive and disciplined.

“He has always been a central midfielder. It is where he feels most confident,” his manager, Diego Simeone, said. “We played him left or right according to our needs but this is his position and he has everything: shots from mid-range, the ability to cover a lot of ground, good in the air, defensively strong, intensity. He is hugely important in the present and he will be in the future, as long as he wants to keep improving. He is young and he has the world before him.”

Saúl has been central to a recovery in the second half of the season that takes Atlético to territory they have trodden before. It has taken Saúl there, too. If Atlético must overcome familiar opponents to make the final, Saúl has already overcome familiar obstacles of his own. In the round of 16 they returned to Leverkusen and afterwards he revealed what had happened since his last visit. “For the last two years I played with an internal catheter,” he told Uefa. “I urinated blood after every training session and every match. I had to put my own health at risk to defend these colours and fulfil my dream. I gambled with my health, driven by the desire to play for Atlético. People don’t see that and don’t appreciate it.”

Waiting in the team hotel to embrace him and his father, José Antonio, when they arrived in Germany was the doctor who had operated on him. Waiting at the stadium were the memories. “I remembered it all,” he said. “How it happened, where I fell, how many times I vomited, the way my body went through convulsions, which hospital bed I was on, which chair I sat in, which team-mates came.” And then he scored a goal that he describes as a “brutal liberation” to help Atlético through.

“I felt relieved. It took a weight off my mind and the fear went away,” Saúl said. “It has been hard for me and my family and it was emotional scoring here. It was a special game. I came back to the place where I could have fallen.”