“I was able to say, 'Happy Birthday' and he was able to smile,” Zuma told reporters after visiting Mandela on Thursday.
South Africa's first black president and anti-apartheid hero has been in the hospital since last month with a recurring lung infection.
For weeks, South Africans have waited and worried. But hopes rose Wednesday evening when Mandela's daughter, Zindzi Mandela, told British Sky News that Mandela's health had improved dramatically and that he might return home "any time soon."
Zuma was a among numerous relatives and dignitaries who visited Mandela in the hospital Thursday morning. Emerging from the parking lot at one point, Zuma joined throngs of well wishers for a brief tribute. Looking grave, hand on heart, in front of an honor guard, he joined the crowd in singing South Africa's national anthem and "Happy Birthday" as a brass band played.
South African Defense Minister Nosiviwe Mapisa-Nqakula, who joined Zuma outside the hospital, told reporters that Mandela’s improvement was remarkable.
"He opens his eyes and nods, as if to say: 'I'm here with you and appreciate what you're doing,'" she said. "We never knew on this day Madiba would be looking at us and be smiling and alert. We want to thank the doctors because we owe everything to them."
South Africans were relieved to note that a statement issued by Mandela's office did not use the word "critical" to describe the elder statesman's health. Previous statements have described his condition as critical but stable.
"Madiba remains in hospital in Pretoria but his doctors have confirmed that his health is steadily improving," Zuma said in the statement, and using Mandela's clan name.
“On behalf of government and all the people of South Africa, we wish Madiba a joyous 95th birthday. We are proud to call this international icon our own as South Africans and wish him good health."