Makuhari - Ireland may be the number-one ranked team, but head coach Joe Schmidt is taking nothing for granted as he prepares to face Pool A rivals Scotland and Japan at the Rugby World Cup.

The Irish take on Scotland in Yokohama next Sunday in a marquee opening fixture that will go a long way to determining the fate of the group -- and possibly the whole tournament.

And while Ireland have had the better of their Celtic rivals lately, Schmidt told reporters on Sunday they had been "a little bit fortuitous" in recent encounters.

"They've always been really competitive. While we've managed to get our nose in front, it was a little bit fortuitous last time we played them," said Schmidt, with Ireland running out 22-13 winners at Murrayfield.

New Zealander Schmidt highlighted the danger posed by Scottish fly-half Finn Russell who "brings an incredible variety to his kicking and passing game and is a threat with his running as well" as well as the "electric" full-back Stuart Hogg.

"You could go right through their team and I think they will look at their personnel and our personnel and rate themselves a smashing chance," said Schmidt.

The coach will not be taking Japan lightly either, as the hosts bid to reach their first ever quarter-final.

He compared the two teams' recent performances against England, which Ireland lost by a humiliating 57-15 and the Brave Blossoms by the narrower margin of 35-15.

"I thought their first half in England at Twickenham was a lot better than our first half in England at Twickenham recently and that is probably a benchmark for them. I thought to lead England 15-10 was a very impressive effort," he said.

He added that Japan were also "a lot more competitive than the score suggests" in their recent match against South Africa, which the Springboks won 41-7 to atone for their shocking defeat in the 2015 World Cup.

Ireland come into the World Cup in Japan off the back of two wins against Wales and desperate to advance further than the quarter-finals for the first time ever at the global showpiece.

Captain Rory Best said what set this team apart from previous Ireland squads was "its collective nature".

"Potentially there had been an over-reliance on one or two great players, world-class players, to produce something magical and if that didn't happen, we were under pressure," said the Ulster hooker.

"I feel this time, we have transferred that load. Yes, we have a few world-class players in positions but definitely our greatest strength is our collective and we know how important each person is to another," added the 37-year-old.

The forecast for Sunday's clash with Scotland is for torrential rain, Schmidt said, admitting this could have some impact on selection and tactics.

"We'll see how we adapt and cope with those different conditions," said the coach.

However, in the final run-up to the tournament, the team will be training in temperatures up to 32 degrees Celsius, he said.

"That's not really Irish temperatures!"