Du Plessis has not played since suffering a back injury during a one-day international against Bangladesh in October.
He said he was still feeling some discomfort and would only play if he felt he was fully fit and that his progress had been slowed by a virus he picked up a week ago.
"The professionals say that if there is still pain in the disc it's a little bit dangerous so that's why we're seeing how it works today," the 33-year-old said.
"Last week I would have said my chances were 80-20, right now it is about 60-40."
Du Plessis acknowledged that the match against Zimbabwe was a stepping stone towards a "big summer" which includes three Tests against India and four against Australia.
He hinted strongly that he was setting his sights on being fully fit for the first Test against India, starting in Cape Town on January 5. "I'd rather feel good and play no cricket rather than play and not feel good," he said.
While the Zimbabwe Test offered South Africa a chance to "get ourselves running again", Du Plessis admitted that India loomed large.
"We're already thinking about the balance of the team and what will be best for beating India."
Du Plessis admitted that a selection dilemma would be eased if he didn't play. "It gives someone else an opportunity to play, which was quite a headache in the first place."
Fast bowler Dale Steyn, who is making a comeback after more than a year out of action with a broken shoulder, is another doubt, with Du Plessis saying he would not be rushed back into action.
"With Dale, in terms of the amount of overs he's bowled leading up to the first Test I think it's important to assess how ready he is to bowl at full intensity," said Du Plessis.
"We want him to be 100 percent. If he we feel he isn't quite there, there's still time to get him ready between now and the first Test against India."
Du Plessis said that a stand-in captain would have to be chosen if he didn't play.
He didn't offer an opinion but the choice probably rests between AB de Villiers, who captained the side when he last played Test cricket in January 2016, and opening batsman Dean Elgar, who was captain in the first Test in England earlier this year when Du Plessis had returned home for the birth of his first child.
Du Plessis said South Africa's first home day-night Test, and the first in the modern era to be played over four days, put a premium on tactics.
"There's more room for thinking out of the box and seeing if you can outsmart the opposition."
He added the use of a pink ball under lights would give heavy underdogs Zimbabwe reason for hope.
"The great thing for Zimbabwe is that they have some senior players back. With the pink ball they will probably see themselves offering more of a challenge because the pink ball does a little bit more.
"The game is still played 70 percent in the day. The ball does something when it's new but then it goes a bit softer, then at night it does a bit more. But there will still be good batting conditions for a lot of the time."
First Published: December 25, 2017, 6:03 PM IST