Zuma's spokesman Mac Maharaj could not say if the ailing 94-year-old Mandela would spend a third night in hospital since being airlifted to a Pretoria hospital at the weekend.
"That (decision) is still in the hands of the doctors. They are saying they doing tests, I don't know what tests. They are saying they are doing further tests," Maharaj told AFP.
Mandela was said to be very comfortable following a visit to the hospital by Zuma on Sunday.
The anti-apartheid hero became South Africa's first black president in 1994 after 27 years of incarceration. He left office more than a decade ago and has since retired to his rural childhood village of Qunu in the southeast of the country.
He has become increasingly frail since retiring form public life, but retains a prominent place in the national psyche.