Brazilian teenager Joao Fonseca on Saturday pulled out of a second tournament in a row with back pain, casting a shadow over his Australian Open build-up.
The world number 29, who won titles in Basle and Buenos Aires last season, withdrew from the Adelaide International next week having also missed the Brisbane International.
The 19-year-old said: "It was difficult to make this decision. After some days of practice I was feeling better.
"But I can't say I'm at 100 per cent. I'm trying my best to recover. The Australian Open is the main goal."
He added, "We did an MRI (scan) and the problem is nothing very serious. But we want to be 100 per cent safe. We know it could have been worse."
The Australian Open starts on 18 January.