Japan's famous win over South Africa has Johnny Sexton slightly worried
Johnny Sexton believes Japan's shock victory over South Africa has handed Ireland a World Cup "alarm call".
Japan's 34-32 victory over double World Cup-winners South Africa in Brighton lit up the opening weekend, shrewd coach Eddie Jones masterminding the greatest upset in the competition's history.
Fly-half Sexton believes Ireland's 50-7 victory over Canada in Cardiff looks even more impressive in the context of Japan's stunning triumph.
Ireland face Romania at Wembley Stadium next weekend, with Sexton calling on Joe Schmidt's men to maintain their focus.
"That victory for Japan has given the sides that are the so-called minnows a boost, to see Japan do that," said Sexton.
"You saw Uruguay do that against Wales, they put it to them for the first 20 minutes and made life difficult for them.
"Argentina were obviously making life tough for New Zealand too.
"It gives everyone a boost to know anything can happen.
"And it's given us an alarm call I suppose.
"It was great to watch Japan win like that, and it made our performance and our result over Canada a lot better, seeing them beat a team like South Africa.
"We obviously could have been on the end of something similar a couple of hours earlier.
"Our game was really good and we got the result we deserved, but we maybe saw the flip-side of what can happen if you maybe take your eye off the ball and the gap is narrowing."
Ireland motored north from Cardiff to their new base of St George's Park in Burton-on-Trent on Sunday, at the Football Association's purpose-built facility.
Team manager Mick Kearney declared an injury-free squad following the Canada battle, save for the hamstring trouble Robbie Henshaw continues to fight.
Connacht centre Henshaw remains in contention to be fit to face Romania at Wembley, but is yet to return to training.
"Thankfully in relation to yesterday apart from the normal bumps and bruises associated with Test rugby we've got a clean bill of health which is good news," said Kearney.
"Robbie is progressing very well through his rehab and the expectation is that he'd be available for selection for the Romania game on Sunday.
"He hasn't trained yet but the expectation would be that would train this week."
Ireland lost their final two warm-up matches against Wales and England, but almost flicked the switch back to grind back into top gear against Canada.
British and Irish Lions playmaker Sexton said Ireland are starting to feel match-hardened now, and just when it matters most.
"That's what those games are for really, to make sure we're ready," said Sexton of the warm-up fixtures.
"That's why we play them and they are badly needed.
"You can't get match-fit training hard.
"We trained really hard in the summer, but the fitness you need in the match is totally different.
"I felt a lot better at the weekend.
"We're all starting to get our match-sharpness and match-fitness and that's what we want at this time now."