Tokyo - Ireland's jet-lagged players fear sapping heat could be a factor against Japan on Saturday after completing a light training session in scorching conditions.
Coach Joe Schmidt has made six changes to his starting 15 for the clash in Shizuoka, on Japan's Pacific coast, but stand-in skipper Rhys Ruddock predicted on Friday that the visitors would face a severe examination.
"I would imagine it is going to be a serious challenge," he told reporters.
"All week we have been mentally preparing, we know it's going to be horrible and it is going to be challenging," added Ruddock. "It will make what is already a hugely difficult test even harder."
An Ireland side missing key players involved in the British and Irish Lions tour of New Zealand arrived in Tokyo earlier this week from New York after thrashing the United States 55-19.
The second game of their three-match summer tour will be a test of stamina against a Japan side who beat Romania 33-21 last weekend.
Ruddock and Leinster back row team-mates Jack Conan and Dan Leavy expect a real test from Japan's hulking trio of Chiefs flank Michael Leitch, Melbourne Rebels number eight Amanaki Mafi and Sunwolves openside talent Yoshitaka Tokunaga.
"We'd have known a lot about Leitch and Mafi from over the years playing for Japan and performances in Super Rugby," said Ruddock.
"But Tokunaga has really impressed us. He's dynamic and he's really good around the breakdown. He could pose a lot of problems for us."
Ruddock added: "We've done our homework but dealing with it is another thing, so hopefully we can prove we're up to the challenge."
Japan will look to exploit their home advantage with an up-tempo game dictated by scrumhalf Fumiaki Tanaka and fly-half Yu Tamura.
But Ruddock insisted Ireland would be ready for the onslaught in a rehearsal for the 2019 World Cup, when they face will tournament hosts Japan in the pool stage.
"They're a really strong side," he said. "We actually know a lot about their players. Guys like Tanaka and Tamura are quality operators. But the work we've done in the week has allowed us to know what their strengths."
Japan have never beaten Ireland and the last time they met was in 2005.
But the Brave Blossoms have made major strides since, shedding their tag as World Cup pushovers two years ago by winning three games under Eddie Jones, including a stunning upset of South Africa.
"It's important that we stick to our plan," Leitch told local media.
"It's a game we have to win so we want to put on a strong Japan