The world's top rugby nations are poised for the start of the Rugby World Cup with the outcome as uncertain as any of the previous eight editions.

New Zealand have won the past two World Cups, but Ireland are currently ranked as the best team in the world.

A resurgent South Africa won the Rugby Championship earlier this year, while Wales took the Six Nations in March.

England have won 10 of their last 14, while Australia's minimum target is a repeat of their 2015 run to the final.

Argentina, semi-finalists in two of the last three tournaments, talented Scotland, Fiji and France teams and hosts Japan are among those hoping to derail the title ambitions of the leading contenders.

"I think this is the most open World Cup we've had for a long time. There are six or seven teams capable of winning the World Cup," said Wales coach Warren Gatland.

"You always need a little bit of luck. You get to the quarter-finals and then take it one game at a time."

North to be exposed once again?

If one of the home nations is to bring home the William Webb Ellis Cup for only the second time, and the first since England landed the prize in 2003, they will need to break a southern-hemisphere stranglehold.

Between them New Zealand, South Africa and Australia have won the other seven titles since the inaugural event in 1987.

Over that time England, Wales and Scotland have filled only seven of the 32 semi-final spots on offer, with Ireland never progressing beyond the last eight.

At the last Rugby World Cup in 2015, eventual winners New Zealand, Australia, South Africa and Argentina contested the semi-finals.

However, the last time the two hemispheres' best met - during 2018's autumn internationals - there were indications that this year's tournament might be more closely contested.

Admittedly with home advantage, England beat South Africa, Wales beat Australia,Ireland beat New Zealand and Scotland beat Argentina on successive weekends.

The home nations have been helped by knowledge from overseas with New Zealander Gatland leading Wales, compatriot Joe Schmidt in charge of Ireland and Australian Eddie Jones coaching England.

Scotland's Gregor Townsend is building on foundations laid by another Kiwi - Vern Cotter - who he took over from in 2017.

New Zealand aim for historic treble

After becoming the first team to successfully defend the World Cup in 2015, New Zealand are aiming to land a historic hat-trick in Japan.

They begin with a stern test of their credentials, facing second-favourites South Africa in their Pool B opener on Saturday.

"We're in a good space. It's an exciting time, it's going to be a massive occasion and a massive game. The South Africans are going to be well and truly up for it so we can't wait," said captain Kieran Read.

After losing just three of 34 Tests in the wake of their 2015 World Cup win, the All Blacks have lost three of the last 13 running into the World Cup.

However, head coach Steve Hansen believes the All Blacks' experience of the business end of the tournament could prove decisive.

"We live there all the time. For some of these teams it's going to be the first time they're going to turn up and feel that pressure. It can be overwhelming when you haven't had it before," he said.

A brave new world for World Rugby

Japan is the first country in Asia and the first outside of rugby union's traditional strongholds to stage the sport's premier event after seeing off competition from South Africa and Italy to host it.

Special considerations have been made to avoid potential culture clashes with extra beer supplies laid on for visiting fans and precautions taken by teams to avoid causing offence with tattoos, considered anti-social because of criminal connotations in Japanese society.

Bill Beaumont, former England captain turned World Rugby chairman, says he is convinced the tournament is going to change both Japan and rugby.

"Rugby mania is going to captivate this nation," he said.

"After 10 years of meticulous preparation the wait is over and the stage is set for what we believe will be a transformational tournament."

World Rugby claim that 1.8m more people, including more than one million in Japan itself, have started playing rugby in Asia over the past three years.

Japanese fans look set to embrace the tournament, with 15,000 fans turning out to watch Wales train in Kitakyushu on Monday and 97% of match tickets sold.

And a win for the Brave Blossoms in Friday's opening match against Russia would give the tournament the perfect start.