"I'm very happy I've made such a significant improvement physically and technically during the competition," Ronaldo told reporters
"Patience is the key word. I managed to stay calm and patient in all the difficult moments."
Ronaldo's second goal equalled Gerd Mueller's record tally of 14 in World Cup finals for West Germany.
Brazil coach Carlos Alberto Parreira also backed Ronaldo, top scorer in the 2002 tournament with eight goals.
"Ronaldo is not in the best possible physical shape but step by step and little by little he is getting there. I'm sure he will be better as each game goes," said the coach.
"We always believed in Ronaldo. He is a special player.
SHOTS BLOCKED
"We deserved to win. We had about 25 shots blocked and we deserved to come top of the group," said Parreira after Brazil maintained their 100 percent record.
Ronaldo came into the tournament after an injury-plagued season with Real Madrid, playing his last game for them on April 8.
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Ronaldo has also became exasperated at continued suggestions he is overweight.
"I have total knowledge of my body, my possibilities, my condition," he said. "After 45 days without playing, I knew I would have to suffer a bit. "
Ronaldo was twice frustrated by Japan goalkeeper Yoshikatsu Kawaguchi in the first 20 minutes.
He finally broke through when he was allowed a free close-range header in first-half injury time to level the score at 1-1.
Ronaldo then wrapped up the 4-1 win with a clinical finish from the edge of the area late in the game.
Japan's coach Zico, a Brazilian, was unhappy his team went out of the competition.
"It's a very disappointing result to take but the way we lost the first game against Australia shows what a tough business football can be," said Zico.
Japan lost 3-1 to Australia, conceding all three goals in the last few minutes.
"For us a victory tonight would have been decisive but Brazil played well and exploited our uncertainties and weaknesses well," he added.
