Japan defeated South Africa four years ago and Ireland great Brian O'Driscoll believes they could provide another Rugby World Cup upset.
Brian O'Driscoll has warned Ireland that Japan are capable of following up their 2015 Rugby World Cup upset of South Africa with a repeat performance on home soil.
Tournament hosts in 2019, Japan provided one of the greatest shocks in World Cup history four years ago with a stunning last-gasp 34-32 defeat of the Springboks in their opening fixture.
The Brave Blossoms are in Ireland's pool this time and former star O'Driscoll is wary of another surprise result when they meet on September 28.
He believes the memory of that South Africa win and the backing of the home support could make Japan a dangerous opponent.
"Of course it is [a potential banana skin]," O'Driscoll, a Land Rover ambassador, told Omnisport. "Speaking to people who know about Japan, they absolutely pose some threats to any team.
"They'll have a new-found confidence at the Rugby World Cup remembering what happened four years ago against South Africa. Sometimes, when you break that seal and beat one of the biggest Tier One nations, the next one might be difficult.
"With the home support, playing like their lives depend on it, it will make them a very difficult opponent. I think they will absolutely be given huge respect.
"Are they capable of causing an upset at the World Cup? Of course they are, because they've done it previously."
Ireland must also contend with conditions that will suit Japan and other Southern Hemisphere sides, but O'Driscoll is confident they have prepared well.
"That's definitely been a focus," he said. "I was looking at the fixture list and certainly the home warm-up games [wins over Italy and Wales] and the timing of them.
"I know, commercially, it makes sense to have games later on, afternoon, early evening, 5.30, 7pm. But the home games have been fixed for two o'clock to try to acclimatise to Japan as much as possible.
"It's those small little details which help you in trying to get your body right for the shock. The only thing they won't be able to plan in advance for [is] the humidity they're going to face.
"But everyone's going to have to deal with that. Obviously certain countries will get it a lot more - South Africa would be well used to huge levels of humidity - but it's going to be the same conditions for every team.
"It's a matter of who's done all the hard work and is able to deal with the pressurised situation in which they find themselves in the pool stage."