0213 GMT July 21, 2019
The Blue Samurai reached the last 16 at last year's World Cup but had to fight back from a goal down at halftime in an absorbing Group F clash in Abu Dhabi, AFP reported.
Turkmenistan captain Arslan Amanov put the plucky underdog ahead after 26 minutes, unleashing a ferocious long-range drive that arrowed into the top corner.
But Osako produced a smart turn and finish inside the box to equalize for Japan 10 minutes into the second half following sustained pressure from the four-time Asian champion.
The Werder Bremen striker put Japan in front four minutes later, tapping into an empty net after Yuto Nagatomo's dinked pass over goalkeeper Mammet Orazmuhammedow.
Ritsu Doan added a deflected third after 70 minutes as Turkmenistan began to wilt.
But Ahmet Atayev swept home a penalty 11 minutes from time to ensure a nervous finish for Japan, which are now unbeaten in six matches since Hajime Moriyasu took charge after the World Cup.
The Japanese are seeking to make up for their Asian Cup flop four years ago when they lost on penalties to the UAE in the quarterfinals.
But with a little luck Turkmenistan could easily have inflicted a major shock.
The Emeralds should arguably have been three up by halftime after Mekan Saparov headed over from pointblank range and an Atayev rocket forced Japan goalkeeper Shuichi Gonda into a brilliant reflex save.
"I was actually surprised how well we did in the first half," said Turkmenistan coach Yazguly Hojageldiyew.
"We lost our way a bit in the second half, but we're not out of it yet. We'll give everything in the next two games."
Japan, only kept out of the World Cup quarterfinals by a stunning Belgium comeback from two goals down, has now lost just one of its last 29 games at the Asian Cup, excluding penalty shootouts.
But it will need to improve if Japan is to threaten the favorite Iran – 5-0 winner over Yemen earlier this week – and a South Korea side soon to be boosted by the arrival of Tottenham forward Son Heung-min.
"The first match is always a bit special and we expected a battle," said Moriyasu.
"Obviously we're happy to get the three points but we have a lot of things to work on for the next game."