All is up for grabs as coach Ange Postecoglou embarks on the latest round of musical chairs for a role seemingly endlessly on the line.
Neither are regulars for their respective clubs - Smith at Bournemouth and Gersbach at Rosenborg - and the chosen one won't find out if he's starting in Saitama until the morning of the match.
Either way, Gersbach won't be complaining as he chases his fourth cap and aims to build on a compelling performance in the 1-1 draw against Cameroon at the Confederations Cup, where he won the spot-kick which earned the Socceroos a point in St Petersburg.
Such is the uncertainly surrounding Postecoglou's picks for the problematic position, Gerabach, 20, wasn't selected for the final group game against Chile in Russia in June.
And to confound matters further, Aziz Behich, who started instead in the 1-1 draw with the South Americans, didn't even make the squad for Japan.
"It's a tough one at the moment ... but I am just happy to be called up and hopefully I can prove I deserve to be here," said Gersbach.
"I have the experience of the Confederations Cup behind me now and that just makes me more hungry to help Australia qualify for the World Cup and be able to go back to Russia again next year.
"It's good that Ange can count on his whole squad to come in and do a job when called upon."
Gersbach has played 10 of defending Norwegian champions Rosenborg's 20 league game this season and arrived in camp off the back of a 79-minute performance in the domestic cup against Valerenga at the weekend.
Smith hasn't featured in any of the Cherries' opening three EPL games of the new season, but did get 90 minutes in the 2-1 league Cup win at Birmingham City.
Having been a regular for his club last season as they romped to the domestic double, Gersbach admits to a level of unease at losing his once automatic starting spot to January signing Birger Meling.
"It's a bit difficult not playing every week," admitted Gersbach.
"I have played a bit over the last month or two but it's hard not knowing when you are going to play and things like that.
"It can be a bit frustrating not being able to cement a starting spot here at Rosenberg.
"But that's just the way football is. I have been training hard and when I get my chance I have been happy with the way I have played.
"There's not much more I can do at the moment. I will just keep working hard and when I get my opportunity do my best to take it.
"I want to stay with Rosenberg and do well with the club.
"We have just qualified for the Europa League, and it's going to be a big few months before the end of the season.
"Hopefully that means more game time for myself with all the matches coming up."
Gersbach is in regular touch with Smith by text, and whoever gets the nod against Japan will have the full backing of the one who misses out.
"I guess it's probably between me and Brad for a starting spot, that looks like the logical thing. But you never know," he added.
"I rate Brad highly, he's a great player and he has it all for that left wingback role ... it really suits him.
"If Ange goes with Brad for the Japan game, I'll be behind him and I'm sure the same will apply for me if I get the nod.
"The Japan game is extra special. It will be a packed house, and there is so much riding on the game for both teams."
Australia's task has been made a little easier with Saudi Arabia's shock 2-1 loss to UAE.
That means Australia could afford the luxury of losing in Japan and still retain a chance of automatic qualification for Russia 2018 by beating Thailand in their final Group B game on September 5.