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Featured Image Credit: The Overlap & Alamy
Guardiola took the Barcelona job in 2008 and went on to win three consecutive La Liga titles and two Champions Leagues.
Mourinho, meanwhile, would find himself in the Madrid job in 2010 and two years later, he managed to clinch the title and finish above Barcelona by nine points.
That season would see Guardiola mentally burn out and he stepped away from football for a year.
Pique was in that dressing room and thinks his manager took on 'too much' and focused on Mourinho's off-field antics.
"We were winning everything at the time and I remember the first time Mourinho came to the Camp Nou he lost 5-0 against us," he told Gary Neville on The Overlap.
"It was a shock of reality of 'woah, these guys go hard'. Mourinho in the press conference, he was every day... you know his style. I think Guardiola at some point, it was too much. It was more important what was happening off the pitch instead of on it.
"I remember at the end of the day, Guardiola left, Mourinho had won the league that year, and all of a sudden he decided to leave. I'm sure part of it is, with Mourinho, it was too much."
A year prior, Barcelona defeated Madrid in a Champions League semi-final to book their place at Wembley against Manchester United.
Pique says Guardiola gave a spectacular post-match interview, but it stopped being about football.
The 35-year-old continued: "I remember the semi-final of the Champions League in the Bernabeu, he did an amazing press conference but it was not about football.
"He enjoys talking about what is happening on the pitch and here, there was a moment where the press conference was focusing on what was happening outside the pitch."
Pique thinks Guardiola went down the route of mental mind games in an attempt to outsmart Mourinho.
In the meantime, Mourinho managed to get into the head of his Los Blancos charges so well, they even ignored Barcelona players on international duty.
At the time, Spain had won Euro 2008 and the 2010 World Cup and were en route to winning Euro 2012.
"Since he arrived, he knew that on the pitch they were weaker than us," Pique added. "We had a better team for sure, and even the relationships between players.
"I remember going to the national team and after those games it was tough because Mourinho goes to the mind of the player and he says ‘these guys hate you’, then you believe that.
"I was in the dressing room of the national team and went to Iker Casillas 'hey Iker' and the guy did not talk to me.
"At that time I did not know, but it was the coach he really knows how to go into the mind."
They'd square off once again in England years later, with Guardiola the manager of Manchester City and Mourinho being in charge of Manchester United and Tottenham Hotspur.