Marcelo Bielsa was there at the start and he will be there at the end. Back in October 2006, before Pep Guardiola took his first steps as a coach, one of the men from whom he asked advice was Bielsa. Should he really become a football manager and, if so, how? Together with his friend the film director David Trueba, Guardiola went to Argentina on what turned into a kind of pilgrimage, travelling 11 hours to seek the wisdom of the man they call El Loco. Late into the night they spoke. Guardiola and Bielsa connected.

Legend has it that Bielsa asked Guardiola: "Do you really like blood that much?" Guardiola presumably decided that he did, just not that much. He became a coach and the most successful coach in Barcelona's history. He has won 13 titles, including three successive leagues, two European Cups and two World Club Cups. But four years later, he bows out, exhausted. The football has taken its toll and so has everything else: the relentless demands, the public profile, the battles. The blood.

This has been a difficult season. It has also been Barcelona's least successful under their manager. They have won only the Spanish Super Cup, European Super Cup and World Club Cup. On Friday, in Guardiola's last game as coach, they could also win the Copa del Rey. The man against him – Marcelo Bielsa, is in his first season at Athletic Bilbao.

Johan Cruyff, Guardiola's greatest mentor, describes this as the end of an era. As farewells go, it is pretty well perfect. Two of the three clubs that have spent their entire history in the first division, the two with the most Copa successes in their history – 25 for Barcelona, 23 for Athletic. Two clubs with a special identity; Catalans and Basques facing each other in Madrid. Guardiola once admitted that he would love to coach Athletic one day; when Barcelona looked to replace him, Bielsa was one of the names on everyone's lips.

On Wednesday, Barcelona's sporting director Andoni Zubizarreta, a former Athletic Bilbao goalkeeper, noted: "Bielsa would adapt well to our club." Javier Mascherano and Alexis Sánchez have played under him at international level.

If there is currently a certain mutual admiration between the clubs, between the coaches it is even greater. When Guardiola announced his departure Bielsa described it as "a huge loss for football." Guardiola described Bielsa's arrival in Spain in similarly glowing terms. La Liga had become richer for El Loco's presence. "I would have liked to have played under him: he is different to everyone else," Guardiola said of Bielsa. "Under him, Barcelona are an innovative artistic expression that's generated a culture – a counterculture," says Bielsa of Guardiola.

In the aftermath of Barcelona reaching the Copa del Rey final, Guardiola became most alive when he was asked about the opponents that awaited. "You can see Bielsa's hand at Athletic," he said. "This will be a fascinating final." He was almost open-mouthed as he talked about them and the astonishing intensity, the purity of their performance. There is no speculation and no gamesmanship, just a relentlessness to their pursuit of the opposition's goal, a generosity of spirit that the Barcelona coach admires. There was a kind of wonder about Guardiola and the fact that he was speaking in English gave it even greater charm: "They run up, they run down, they run up, they run down, they run up, they run down …"

When the two teams met at San Mamés earlier this season, it finished 2-2. Guardiola called it a canto al fútbol, an ode to football, a love song to the game. At the end, the two men embraced. "Your players are beasts," said Guardiola. "So are yours," replied Bielsa. It was moments like these which made the blood worthwhile. When the final whistle goes on Pep Guardiola's final game as Barcelona coach, they will embrace again. He has come full circle.