Kobe - Ireland number eight CJ Stander said the shock loss to Japan was one of the worst days of his career but has vowed to "go again" against Russia on Thursday.

Although being sent off against his native South Africa was "probably always going to be at the bottom", Stander said Saturday's 19-12 upset in Shizuoka was "close to the bottom" of his career.

Japan's stunning victory threw open Pool A -- which also includes Scotland and Samoa who play on Monday -- and lit up the tournament, bringing back memories of the "miracle of Brighton" when the Brave Blossoms beat the Springboks 34-32 in 2015.

"Now we've got to look after the ball, make sure we stick with our process when we set down the attack plan, make sure we back what we're doing," Stander told reporters on Monday.

The 29-year-old has started both of Ireland's games so far and stood out in the 27-3 opening defeat of Scotland that marked the Irish out as one of the contenders for the title.

He may face the Russians in Kobe as well after fellow back row forward Jack Conan was ruled out of the tournament with a broken foot, replaced by Ulster's Jordi Murphy.

But the "excitement of the World Cup" meant he had no qualms about potentially playing three games in 11 days.

"This is what you want to do when you're growing up, thinking about being part of a World Cup.

"So yes, recovery-wise we're being looked after really well. We've got training on Tuesday, so we've got to make sure the body's ready.

"So if I'm selected I'm good to go again."

Murphy could be pitched into action soon after his arrival in Japan but Stander said the Ulsterman "always fits right in."

"He's going to bring good balance and a lot of energy to the back-row, with his ball-carrying. So we're looking forward to having him in there."