Ayase Ueda warned Sweden his confidence was sky-high after the red-hot striker scored a classy World Cup brace to put Japan on the cusp of the knockout rounds.
Japan thrashed Tunisia 4-0 on Saturday to join the Netherlands at the top of Group F and all but seal their place in the last 32 with a game to spare.
That game is against Sweden, who sit third after being hammered 5-1 by the Dutch in Houston.
In the landmark 1 000th match in the history of the World Cup, Japan put on a show in Mexico's Monterrey to demonstrate why they are fancied to go far in the tournament.
Netherlands-based Ueda, who has been linked with a move to the Premier League, scored from outside the box in the first half and got his second near the end with a clever header.
Daichi Kamada and Junya Ito got the others against an outclassed Tunisia, who cannot now advance.
"I'm genuinely happy. It feels as if what I've built since feeling that frustration four years ago paid off," the 27-year-old Ueda told reporters.
The forward played just 45 minutes at the 2022 World Cup as Hajime Moriyasu's side bowed out in the last 16 to Croatia.
But he arrived in North America off the back of banging in 25 goals in 31 appearances in the league for Feyenoord.
He topped the scoring charts in the Dutch top division, reportedly igniting interest from several English clubs including Tottenham.
His clinical double against a wretched Tunisia were his first World Cup goals.
"(It) felt totally different to all the ones I had scored before in terms of joy, sense of accomplishment and the weight of responsibility I've been carrying," he said.
"The quality of my play is different from four years ago, and I'm playing with confidence."
Japan, who beat England at Wembley in the lead-up to the World Cup, twice fought back to hold the Netherlands 2-2 in their opening match.
They next face a Sweden side on Thursday in Dallas who beat Tunisia 5-1, only to go down by the same scoreline to the Netherlands.
With even third place in the group possibly enough to reach the next round, Graham Potter's Sweden are still in a strong position to progress.
But speaking after the thumping in Houston on Saturday, Netherlands skipper Virgil van Dijk warned Sweden that Japan presented a huge challenge.
"Japan are a very disciplined side, they keep it very compact and tight," said the Liverpool defender.
"Sweden will have a very, very tough job ahead of them, that's for sure."
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