With just three rounds remaining, the Reds languish seven points adrift at the foot of the ladder following last weekend's 26-13 defeat to interstate rivals the Waratahs in Sydney.
The poor sequence under coach Eddie Jones has tested Tune's reserves further still with the 30-year-old revealing his enthusiasm for the game was already declining following a punishing career.
But the Brisbane-born star maintains he has never once entertained thoughts of postponing his retirement as he edges closer towards drawing the curtain on a 12-year career with the Reds and the Wallabies.
"No, none whatsoever," said the 1999 World Cup winner. "I've been planning for this for a fair while. Towards the end of last year I knew that the end was coming fairly quickly.
"The body had had enough and as a result my love and passion for the game was diminishing as well.
"Mentally and emotionally it's been a pretty difficult 10 or 11 weeks for that reason."
The 46-Test veteran admits that being on the receiving end has made for an even more difficult finale.
"I've never had to get myself up so much for every single game like I have this year, and when you're losing it's even more difficult," he said.
"I'm just not enjoying myself any more and I think if we were winning it would be a little bit more enjoyable but physically I'm spent."
Reds assistant coach Damon Emtage paid tribute to Tune, hailing him as one of the greatest-ever players for club and country.
"He's going to be very hard to replace in more ways than one," said Emtage. "He's one of the greatest players ever to wear the Reds jersey, and also for Australia, but also what he provides off the field.
"He's a very good role model."
Emtage insisted it was doubtful Tune would stick around after the end of the season despite pledging a long-term interest in the code.
"He has a big interest in the development of rugby and I'm sure that once the dust settles he'll give great thought to how best he can give back into the community," added Emtage.
The Reds face the Cheetahs this Saturday before flying out for a double-header against South African pair the Stormers and the Bulls to finish the season.