Tokyo - It's nice to be recognised as number one in the world but to Ireland coach Joe Schmidt that means nothing when preparing his injury-hampered side to face Scotland in their World Cup opener.

With a rearranged Ireland backline and forecast wet weather, the outcome may well be decided in a titanic battle of the halves.

Ireland's Conor Murray and Jonathan Sexton face Scotland's Greig Laidlaw and Finn Russell at Yokohama on Sunday.

Laidlaw is effective whether kicking for territory or points, Russell possesses a variety of options with his kicking game but Schmidt said: "When you match that up with us with the likes of Conor Murray and Jonathan Sexton I don't think there's much between the teams.

"Whoever does maybe get the bounce of the ball or can be just a little more efficient than the other it may tip the balance."

The rival halves know every nuance of each other's play from regular clashes in the Six Nations, but Schmidt says the World Cup will surely produce the unexpected.

"There's certainly things you do hold back a bit and I've no doubt we'll see something from Scotland that we haven't seen before because we've been planning this for a long time," said Schmidt, who bows out as Ireland's coach at the end of the tournament.

Ireland, who have never progressed past the quarter-finals at the World Cup, tuned up for Japan with back-to-back wins over Six Nations champions Wales -- which saw them promoted to the top of the global rankings.

Scotland, currently ranked seventh and considered unlucky not to make the semi-finals in 2015, completed their preparations with two wins over Tier-Two Georgia.

It was a workout that suited the Scottish forwards, with coach Gregor Townsend suggesting that could be where he sees an advantage over Ireland.

"We know them well and are well aware of the strengths they possess," Townsend said.

"As we've got closer to the game our training has been more about fine-tuning the hard work that's gone in over the pre-season into a focus for Sunday's game."

If weather and ground conditions permit, there is interest in the outside backs where Ireland's preparation has been hampered by injury.

Rob Kearney, Keith Earls, Robbie Henshaw and Joey Carbery are all missing although, Henshaw apart, they should be available for their next match against Japan.

Scotland, however, are primed with players who have won British and Irish Lions honours including a seasoned back three of Stuart Hogg, Sean Maitland and Tommy Seymour.

Under Schmidt, Ireland have won six of their last seven games against Scotland but apart from a 40-10 blowout four years ago the scoreline has been relatively close including a 22-13 victory this year.

For the New Zealand-born Schmidt, factors such as the closeness of recent results and Ireland's injury problems put the relevance of world rankings into perspective.

"It's the first time we've ever held that ranking so it's something that we're proud of," he said.

"But the World Cup, more than anything it's not about being the best in the world it's about being the best on the day and that's one of the things that really excites people.

"You don't have to be the best team in the world or ranked number one or ranked wherever, it's just on the day in the two 40-minute windows that you have the opportunity to put your best foot forward, to be the best on the day."

Teams:

Ireland

15 Jordan Larmour, 14 Andrew Conway, 13 Garry Ringrose, 12 Bundee Aki, 11 Jacob Stockdale, 10 Jonathan Sexton, 9 Conor Murray, 8 CJ Stander, 7 Josh van der Flier, 6 Peter O'Mahony, 5 James Ryan, 4 Iain Henderson, 3 Tadhg Furlong, 2 Rory Best (captain), 1 Cian Healy

Substitutes: 16 Niall Scannell, 17 David Kilcoyne, 18 Andrew Porter, 19 Tadhg Beirne, 20 Jack Conan, 21 Luke McGrath, 22 Jack Carty, 23 Chris Farrell

Scotland

15 Stuart Hogg, 14 Tommy Seymour, 13 Duncan Taylor, 12 Sam Johnson, 11 Sean Maitland, 10 Finn Russell, 9 Greig Laidlaw, 8 Ryan Wilson, 7 Hamish Watson, 6 John Barclay, 5 Jonny Gray, 4 Grant Gilchrist, 3 Willem Nel, 2 Stuart McInally (captain), 1  Allan Dell

Substitutes: 16 Fraser Brown, 17 Gordon Reid, 18 Simon Berghan, 19 Scott Cummings, 20 Blade Thomson, 21 Ali Price, 22 Chris Harris, 23 Darcy Graham