Yuki Otsu's 34th-minute header earned the victory, but Takashi Sekizuka's side should have extended their lead through both Kensuke Nagai and Hiroshi Kiyotake.
In truth, Spain were sluggish throughout with Inigo Martinez's sending-off at the end of the first half only worsening matters, but the VVV Venlo centre-back commended his team's game-plan.
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"The beginning of the game is very important and I told them from the beginning that we could play like that and we could score the first goal," Yoshida told reporters.
"Otsu's strike showed we pressured from the front and I think it makes it very tough for them; also, they had one fewer player.
"We could have scored more - so we have to [take those chances] - but we're good defensively.
"But we can't be satisfied with the result. [We] go on from here with the game against Honduras and Morocco and play better than today."
Yoshida represented his country at the previous Olympic Games in Beijing, with the team losing all three of their group games, and the captain was quick to dismiss any comparisons between the 2008 team and the one on show on Thursday.
"The team is totally different. Maybe four years ago, only a few players played in Europe," he stressed.
"Now many [of our] players play in Germany, Netherlands and other countries, so I think we have more confidence than four years ago."