Du Plessis has not played any cricket since injuring his back against Bangladesh in October and confirmed on Sunday that he has also been laid low recently due to a virus.
The 33-year-old told reporters at St Georges Park: "My progress was really good over the last two weeks.
"(But) I picked up a virus last week which has slowed me down a little bit, so right now where we stand is trying to get to practice and see how (the back) feels.
"I practiced yesterday (Saturday) and there was still a bit of pain in my back, so that is why we will see today (Sunday) how it goes and then tomorrow (Monday) again."
When pressed further on his fitness, especially with a Test series against India starting early next week, Du Plessis gave maybe the biggest hint that he will not be risked against Zimbabwe.
"Last week I was 80-20 for playing, probably now closer to 60-40 playing," said the Proteas skipper.
"As I said, yesterday was still a little discomfort but we are hoping today (in training) I will feel a little better."
The last time Du Plessis missed a Test was in England in July at Lord’s when he stayed home and missed the first game of the series due to the birth of his first child.
Opening batsman Dean Elgar filled in for Du Plessis on that occasion and is favourite to get the job again if the regular skipper is ruled out.
However, former captain AB de Villiers is also back playing Test cricket for the Proteas and could be in contention to lead the side in a one-off situation.
"The selectors will get together with the coach and decide who they think is the best guy to stand-in in a one off Test," said Du Plessis.
Play gets under way on Tuesday at 13:30.