The 27-year-old last played for Japan two years ago and was without a club for six months after leaving Arsenal in July last year to focus on his rehabilitation.
He signed for Ajax in January but was forced to pull out of Japan's squad for friendlies in March after more physical problems.
Japan coach Hajime Moriyasu still named him in his 26-man squad for the World Cup starting next month and Tomiyasu is determined to seize his chance.
"I feel a sense of responsibility because I was picked for the squad even in these circumstances," he told reporters after training with the team.
"I want to repay that faith with my performances on the pitch."
Japan have been drawn with the Netherlands, Sweden and Tunisia in Group F and play the Dutch in their opener in Dallas on June 14th.
Tomiyasu will be pushing for a starting place as Japan look to advance past the last 16 for the first time.
He said his return to the team had been "a long road".
"A lot has happened on the way to me being picked for this World Cup and I don't want to forget that and use it in a good way," he said.
"I want to keep in mind that I can't take being here for granted."
Tomiyasu has 42 caps and played at the 2022 World Cup in Qatar, where Japan stunned Germany and Spain before losing on penalties to Croatia in the last 16.
He said he wanted to make sure he enjoyed this year's tournament in the United States, Canada and Mexico.
"To be honest, I didn't feel that way last time when it was all over," he said.
"I think I was too self-conscious, and this time I want to say something if something needs to be said, and go into the tournament without overthinking anything."