Tokyo - Coach Joe Schmidt said Ireland would not look too far ahead of themselves to a potential quarter-final with South Africa, rating his side as "middleweight" compared to the "heavyweight" Springboks and New Zealand.

Schmidt is targeting a first-ever semi-final for Ireland in Japan and they started on a positive note with a 27-3 demolition of Six Nations rivals Scotland on a wet evening in Yokohama.

The bonus point win puts Ireland in the box seat to top Pool A, meaning they would in all probability face the Springboks in a daunting quarter-final.

Schmidt described Saturday's 23-13 All Black Pool B win over the Springboks as a clash of heavyweights while Ireland "might be light heavyweight or middleweight."

"South Africa were bristling and the All Blacks at times were brilliant," said the coach.

After Japan handsomely won their opening Pool A match against Russia, Schmidt said he could take no chances when naming the side to play the host nation next Saturday but may rest some players after that.

Japan "are a dangerous team and if they get some tempo we might be on the back foot so we've just got to take it step-by-step," he said.

"We won't be talking too much about South Africa but if we maybe can get past Japan, I think we've got Samoa and Russia and hopefully at that stage we can a potentially manage some players."

Ireland already have three key backs out through injury but Schmidt said there were no concerns over half-backs Johnny Sexton and Conor Murray, who would be fit to play Japan despite being replaced early.

There were however question marks over centre Bundee Aki and flanker Peter O'Mahony who both suffered head knocks.

Schmidt declared himself pleased with the opening four-tries-to-nil victory over Scotland, with Ireland able to maintain their first-half momentum despite heavy rain after the break.

"The conditions made it difficult to construct things but we got the only try in the second half when a lot of the replacements were on," he said.

"The scrum that we turned over with the replacements was really impressive.

"I was happy with the number of risks we did take. I know it's only a one-pass play to play up the short side... but it worked really well for us twice."