Tokyo - A dominant display by Ireland's forwards earned them an impressive bonus-point Rugby World Cup win over Celtic rivals Scotland on Sunday, as fellow contenders England also enter the fray.
The emphatic 27-3 Pool A win underscored Ireland's status as one of the pre-tournament favourites and sent out a message to southern hemisphere challengers New Zealand and Australia, who both won on Saturday.
World Player of the Year Johnny Sexton answered his critics after a worrying dip in form as Ireland shrugged off injuries to key players in their back line to seize the initiative in a tight group.
But it was the tight five forwards who broke Scotland's resistance, with pushover tries in the first half from lock James Ryan, prop Tadhg Furlong and hooker Rory Best.
Scotland had lost six of the previous seven games against the Irish but scented an upset in Yokohama as coach Gregor Townsend named the most experienced Test team in Scottish rugby history with an astounding 630 caps in the starting XV.
But heavy rain in Yokohama hampered Scotland's handling as their backs studded with British and Irish Lions failed to fire.
"I think we started really well. We were positive. We wanted to attack. Obviously with the conditions in the second half, it became more of an arm wrestle but we know how good a team they are and we're delighted with the win," said skipper Best.
"That was a good start for us but we know... how much we need to improve going forwards," added Best.
Sunday's clash of the Celts in Yokohama followed an unconvincing 47-22 win by fellow Six Nations outfit Italy against Namibia, the lowest ranked side in the competition.
That match saw Italy's talismanic captain Sergio Parisse make history as only the third man to play in five World Cups but it was an error-strewn game to forget in at times torrential rain and fierce winds.
"We'll have to move on from that pretty quickly. It wasn't very pretty. There's too many errors in it," said Ireland coach O'Shea while Parisse agreed it was "not maybe a really nice match to see."
The last match of the day sees England take to the field for the first time looking to prove that they are serious challengers for the Webb Ellis Cup.
Eddie Jones' side against Tonga includes marauding back-row forward Billy Vunipola, who has Tongan parents and whose father and uncle both played against England at the 1999 World Cup.
Jones has picked his strongest possible side for the opening Pool C encounter under the roof at the Sapporo Dome and is sticking to his twin playmaker strategy with Owen Farrell at inside centre and George Ford at fly-half.
The veteran coach described his side's impending journey in typically colourful terms, comparing it to a rollercoaster.
"We are at the top of the ride now, we are looking down -- everyone's nervous, everyone's excited. You get down the first slope, you are not sure if you are going to throw up or hang on," said Jones.
Farrell said England's rugby team was hoping to build on the success of the country's cricketers, who won the World Cup at home and football team which reached the World Cup semi-finals in Russia.
"Whether it was the football World Cup, Cricket World Cup, you see the momentum that gathers back home and I'm sure those teams were able to feed off that," Farrell said.
Sunday's matches come after three pulsating games the previous day when Australia overcome a feisty Fiji, France pip Argentina 23-21 and the mighty All Blacks beat the Springboks.
A powerful typhoon expected to squeeze through the sea between Japan and the Korean peninsula largely spared the matches, although there were squally moments in the Italy match and rain for Ireland-Scotland.