Felipe Santana said he left Borussia Dortmund for arch-rivals Schalke because he wanted to keep his dream of playing for Brazil in the 2014 World Cup alive.
Santana, 27, this week became the first player to switch between the two clubs since Andreas Moller made the same move in 2000.
Santana, who scored the winning goal in Dortmund's Champions League quarter-final success against Malaga this season, told Bild he had found the decision to leave the Westfalenstadion difficult but needed to be playing first-team football if he is to finally break into the Brazil set-up.
"I had five great years in Dortmund and wish Dortmund only the best," he said. "I will never forget this time. A part of BVB will always remain in my heart. I can only say: Thanks for everything. A new beginning is never easy. That was also the case in Dortmund. But I will fight and give it my all for Schalke – and that's how I want to win the fans' hearts."
He added: "I had a great time in Dortmund, but I need a new challenge and want to make it to the World Cup with Brazil in Brazil. To do so I need to play."
The centre-back, who arrived at BVB from Figueirense in his homeland, said he had spurned the chance to move abroad as he wanted to remain in the Bundesliga.
"I feel great in Germany and have now learned the language. Moreover, I want to apply for German citizenship in a few years' time."
With Santana having departed, Dortmund moved to strengthen their defence with Sokratis Papastathopoulos, who had been expected to join Bayer Leverkusen. The Greece international, 24, said BVB coach Jurgen Klopp had been key to his decision.
"I absolutely wanted to join Klopp," he told Bild. "Jurgen Klopp played an important part in my decision. This club is better for my future, without doing Leverkusen down.
"I did not let Leverkusen down. I had to decide between two attractive clubs. It is important to me to clarify that I did right by Leverkusen. I did nothing wrong."