The New Zealand team, who overcame Tonga 41-10 in their first pool match but won few admirers from the error-strewn performance, watched the 'Brave Blossoms' open their tournament with an impressive display against France on Saturday.
The match was in the balance for much of the second half, only for three late tries to secure a somewhat flattering 47-21 result for the French.
Ellis said the performance sent a clear signal the John Kirwan-coached Japanese would not be intimidated by the traditional powers of world rugby.
"They went really well. I think they are really well coached and they've got the players to execute what they're trying to do," he said.
"They're all in really good positions on the field, they've got good systems. I think their guys know what they're doing in the contact, they carry well and they clean out well.
"You've got guys playing in good competitions all over the world, throughout the year. So when they come together they're really strong, no matter who the team is.
"So we know that about Japan and we're going to have to prepare well if we're going to perform well."
Watching games on Saturday from the comfort of their hotel, the All Blacks could not help but be impressed with the performance of some of the tournament's so-called minnows, with Romania and Namibia also acquitting themselves well against more established opponents.
"Everyone's growing and everyone's learning -- a lot of the time off the same sorts of coaches -- so guys are getting the same game plan and it's getting tougher and tougher," said All Blacks number eight Victor Vito.
"It's our job to keep ahead really.
"Take that Japan game yesterday. If it was just another test match the French probably would have had the better of them for most of the game. (The World Cup) gives the minnows time to shine and they grow another arm and another leg."
Competition for places in the All Blacks team for the Japan match, and the remainder of the tournament, is heating up.
"It's good for the team when a man gets an opportunity in a jersey and he performs," said winger Cory Jane.
"It's better to have to battle for a jersey for a starting spot than to be guaranteed it. I guess if you're guaranteed it you can lapse a few times and take it for granted."
Jane will be feeling the heat after Richard Kahui's two-try, man-of-the-match performance on his favoured right wing in the tournament opener last Friday.
But that failed to curb the noted jokester's cheeky side as he put his own spin on the decision to move Kahui to the left wing when he came on against Tonga in the last quarter.
"I got told to throw him over there. He'd scored two tries, set up four, so may as well see what he can do on the left," said Jane.
"I think they (the coaches) want to see if he can play every position. I think he might be playing first-five (flyhalf) this week as well," he said, tongue firmly in cheek.
On the injury front, Adam Thomson and Kieran Read were recovering well, with the former expected to resume training this week.
Read is unlikely to be fit to return until the All Blacks' last pool game against Canada in Wellington on October 2.