With a boy’s ardor, Messi put Barcelona in the final of the Champions League in Europe — the world’s most prestigious club tournament — to be played against Manchester United on Saturday at Wembley Stadium in London. He delivered both goals in Barcelona’s 2-0 victory in the first leg of the semifinal round against Real Madrid. This gave Messi a startling 52 goals in his first 50 matches of a season in which he also leads the Spanish league in assists. The first goal was merely outstanding in its timing and clever anticipation. The second was a masterpiece of acceleration, power, balance, agility, vision and darting virtuosity.

“I think this genius is impossible to describe,” Pep Guardiola, Barcelona’s manager, said. “That’s why he is a genius. He has instinct. He loves to live with pressure. He is one of the best ever created.”

That defining Champions League semifinal match was played April 27 at Estadio Santiago Bernabéu in Madrid. Nine months earlier, stars from Barcelona and Real Madrid joined to give Spain its first World Cup title. Together, they lifted the winner’s trophy in South Africa. But now they played for club, not country. Temporary brotherhood fissured. Blood rivalry resumed. Madrid, the capital, was once the base of Franco’s dictatorship and is now the seat of Spain’s constitutional monarchy; Barcelona sits in the heart of the autonomous Catalan region, with its own language and cultural (and soccer) identity.

An Argentine, Messi was not born into these tensions. He came to Barcelona at 13, when the club agreed to pick up the costs of treatment for a growth-hormone deficiency. As the story goes, his contract was written on a napkin. At the time, he was about 4 feet 7 inches. He now stands 5-7. If his lack of size made him shy and self-conscious as a boy, his low center of gravity made him spectacularly elusive as a soccer player.

“We thought he was mute,” said Gerard Piqué, the lanky Barcelona center back who played with Messi in the club’s youth academy. “He was in the dressing room, on the bench, just sitting. He said nothing to us for the first month. We traveled to Switzerland to play a tournament, and he started to talk and have fun. We thought it was another person. He was really good, but he was really small and thin. His legs were like fingers. One coach said, ‘Don’t try to tackle him strong, because maybe you will break him.’ And we said, ‘O.K., but don’t worry because we cannot catch him.’ ”