Trulli, the teenage son of ex-Formula 1 racer Jarno, notched up his first win in the opening race of the fourth round of the season from pole position, and doubled up in the final race of the weekend.
He may have made it a clean sweep if he hadn't been disqualified from the second race for his TOM'S car being underweight, as he finished third on the road behind Hibiki Taira and Iori Kimura, who were both penalised post-race.
Yamada said he was impressed by the way Trulli handled the pressure of starting from pole for the first time, as he led every lap in both of the races he won and never looked threatened.
"Winning at Fuji twice shows his potential," Yamada told Motorsport.com's Japanese edition. "It's a difficult circuit where conditions have a big effect.
"When you are leading and there’s nobody in front, it’s easy to drive, but there was never any point when he didn’t look in control, so I think it shows his concentration and mental strength. That’s why he was able to win twice. There are plenty of drivers who would mess up the start [in his situation], but he didn’t.
"If I’m totally honest, if things continue like this, I think we would have to say that Enzo is stronger than [TOM'S team-mate] Taira, even though he is at the top of the standings."
Trulli arrived at Fuji having only finished on the podium once in the opening three rounds of the season, but said he was boosted by Super Formula Lights switching from Hankook to Yokohama tyres mid-season.
"The tyres are quite different," said Trulli. "I was struggling a bit with understeer on the Hankooks, and generally I don't like understeer. But now we are on Yokohama tyres, the car feels easier to drive and I feel I have more control of it."
However, Yamada believes that Trulli will have to change his driving style if he wants to stay racing in Japan for the long term, as his father Jarno hopes.
"I’ve been doing F3 and Lights for a long time, and most drivers who come here from Europe prefer to have an oversteer balance and less rear grip," Yamada explained. "Enzo is the same.
"It will be important for him to change that if he wants to race in Super Formula and SUPER GT, where tyre preservation is important. In Lights, the races are short, so even if you have some degradation it doesn’t matter so much.
"But if we think about the future, I want him to think about how to be fast and kind on the tyres at the same time, while also making good use of the experience he has gained in Europe."
Trulli's victories have put him fourth in the championship with two rounds to go, and second-best of the rookies behind Honda protege Shun Koide.
Yamada suggested that Trulli has shown the potential needed to be considered for a step up to Super Formula in future, even if the 18-year-old still needs to work on his consistency.
"I think we can expect a lot of Enzo [for the future]," said Yamada. "Of course, there are still some things we can’t be sure about. For example, even though first qualifying went well, second qualifying went badly, so there are things he needs to work on. But I think he has the talent."
