"To stay calm like that, from the moment you enter the court to the last point, it's not easy," added the former world number four, who reached the semifinals of Wimbledon in 1996.
"If he continues to develop the way he has done in the past two weeks and remains motivated, he can continue to be the first number one in the world in Japan."
Osaka, a confident Pokémon fan whose playful nature has made her the favorite of the women's tour, is expected to go from 19 to seven in the standings after becoming an unlikely hero in Japan, still shaky after a summer of typhoons and deadly earthquakes. .
The first Japanese player, man or woman to capture a Grand Slam title, Osaka won her first WTA tournament in Indian Wells in March when her trophy presentation went viral: "This is probably the worst acceptance speech of all time" . She blushed.
Osaka, who is of Haitian-Japanese origin and raised in the United States, responds to questions from the Japanese media in English with a subtle Caribbean caricature, often apologizing for his rudimentary Japanese.
But he has firmly put Japanese tennis on the map with his stunning victory over Serena, who was chasing a crown of 24th singles in Grand Slam with the same record.
They fight the fire with fire, the game of Osaka reflects the one of the Americans: great serve, murderous shots in both flanks, especially the blow of right, and a determination of steel.
Data compared Osaka to China's Li Na, who retired in 2014 after winning the French and Australian Open and reaching number two in the world.
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"Osaka is taking the power of women's game tennis with her own power: an Asian player, a Japanese player," said Date, who retired last year at the age of 46.
"Until now, only Li Na had the physique to be able to face that kind of power."
"You could say that Serena distrusted the power of Osaka," Date added.
"And it's still being developed, the best players will study it now and she'll have to go from being the challenger to a position where she has to produce."
The congratulatory messages were flooded for Osaka, including tweets from Kei Nishikori, who became the first Japanese male player to reach a Grand Slam final at the 2014 US Open.
After raising $ 3.8 million in prizes, Osaka's extrajudicial gains will increase 10 times in the coming years, from $ 1.5 million to more than $ 15 million, according to Forbes.
The test will be how he reacts to his sudden fame, starting with the Pan Pacific Open in Tokyo later this month.
But his remarkable balance in Saturday's final is a good omen.
Even when Serena threw a tantrum and called referee Carlos Ramos "thief" after a violation of the code by coaching quickly spread until he scored a point for a shattered racket, and then a game for repeated insults, Osaka kept the calm.
After completing the match, Osaka showed his softer side, breaking into tears while the boos sounded among the fans still outraged by the opponent's setback with Ramos.
Date gave a fulminating evaluation of the crowd's reaction after Osaka clearly could not taste his achievement.
"For the atmosphere to turn like that after the first (main) final in Osaka was a shame," he said. "Thinking how she must have felt to not be able to enjoy her victory and see her cry made my heart ache."