De Bruyne used his time away with Belgium to put an end to speculation about his Napoli future, telling reporters at the squad's training camp that he never seriously considered leaving the club over the summer.
"I never considered the idea of leaving Napoli this summer, I haven't spoken about it with anyone," he said.
The 35-year-old also confirmed his commitment to Belgium under new head coach Mark van Bommel, who replaced Rudi Garcia after the Red Devils' World Cup exit to Spain in the quarter-finals.
"I told van Bommel that I'm here to help. I feel good both mentally and physically, so I just wanted to continue like this. A lot of players have asked me to stay. I don't want to think too much about the future, but for me it wasn't a problem."
On his form after a slow start under Massimiliano Allegri, who had repeatedly left him among the substitutes while he built up fitness, De Bruyne said he has been pleasantly surprised by his own level.
"I think my level is good; I was pleasantly surprised by my form already at the start of August. If it continues like this, it will be fantastic for me. Compared with my peak ten years ago, it might be slightly different, but I think that's normal."
He addressed the fitness questions directly, pointing to his recent run of full matches.
"I feel very good. I started training again at the start of August and felt fit straight away. At the start of the season I didn't play every match, but recently I've played the full 90 minutes three times in a row. With the national team a new project is starting. I see a lot of enthusiasm, we'll see how it goes."
Belgium face Mancini's Italy in Rome on Friday, De Bruyne's first meeting with the country since joining Napoli, and Allegri is expected to hand him a more settled role in central midfield once the international break ends.