The 24-year-old Italian withdrew from the US Open mixed doubles event, where he was to partner Katerina Siniakova, a day after illness forced him to retire five games into his Cincinnati final against world number two Carlos Alcaraz.
Cahill said the Australian Open and Wimbledon champion was suffering from a virus.
"I spoke briefly with him last night. He was feeling a little better," Cahill told ESPN on Wednesday.
"He'll rest, that's the plan, and I hope he can get on the court on Thursday and start hitting some balls. We're confident he'll be fine."
The US Open runs from August 24th to September 7th.
