Ireland head coach Joe Schmidt was relieved to see his side clinch a hard-fought 35-13 victory over Japan at Ajinomoto Stadium in Tokyo.
Tries from Garry Ringrose, Josh van der Flier, Kieran Marmion and Rhys Ruddock established a commanding 28-8 half-time lead, but the visitors wilted in the heat and high humidity after the break.
Ireland were kept scoreless for the majority of the second half, with substitute flanker Sean Reidy adding a late try.
The win completes a clean sweep on Ireland's summer tour following a 55-19 victory over the US Eagles in New Jersey and a 50-22 win against the Brave Blossoms last Saturday.
"Having spoken to the players, that was very, very tough going," said Schmidt.
"The Japanese defence came very quick off the line, very effective in the tackle and made the day very hard work.
"It was very hot out there, the players were fatiguing early in the game, so it was great to get a good start and hang on at the finish."
Ireland were looking on course to bring up another half-century against Japan thanks to another powerful first-half display, but the hosts came roaring back into the contest after the break.
Led by inspirational captain Michael Leitch, Japan were much improved in the second half with Kotaro Matsushima scoring a well-worked try to add to wing Akihito Yamada's earlier effort.
"I think we got exactly what we expected, it was very tough, very physical," noted Schmidt.
"Michael Leitch was enormous. Even when it looked like John Cooney was going to get away (at the end of the game), Leitch got back to tackle him. He does not give up.
"I could mention so many players...Yamada, first time he got the ball he beat our winger so quickly, I think our player hardly saw him.
"The other area is the scrum, they were so much stronger in the scrum, they held us very solidly at scrum time."
Ireland captain Ruddock agreed that his young side had been pushed to the limit.
He said: "Everyone found it unbelievably tough, both in terms of the physicality, speed and the way they challenged us in the tackle, with strength and tempo. It's certainly our biggest test of the tour and we had to work very hard to get the win."
Meanwhile, Japan head coach Jamie Joseph bemoaned his side's lack of discipline during the first half.
Joseph's men conceded a flurry of penalties as they struggled to contain a rampant Ireland in the opening half before their resurgence after the break.
"The penalties that are avoidable for me are the silly penalties when we were on the back foot," he said.
"Under pressure, those sort of penalties you just can't afford to give away at Test match level. Today, they gave away a lot of penalties around the ruck.
"There was nothing wrong with our attention, it was just our application," he said.