The fatigue left him hospitalised in June pushing him to play down reports of health problems.
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Zuma, 72, has noticeably lost weight since coming to power in 2009. He was hospitalised for two days of tests following what his office said was a demanding schedule in the run-up to May's national election.
"I think we did overstretch ourselves, I think there was fatigue thereafter," Zuma said in an interview with national broadcaster SABC that was aired on Sunday.
"There was a period where I really took it easy. I couldn't say my health was in perfect condition -- I'm in perfect condition now -- but at that time, one could feel the strain of elections."
The Sunday Times newspaper reported in June that Zuma's hospitalisation was triggered by heart problems, diabetes and high blood pressure. His spokesman later dismissed the report, saying Zuma was fine.
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Worries about Zuma's health have raised some speculation he may not see out the full five years of his second term. His ruling African National Congress (ANC) won a 62 percent majority in the May polls, its narrowest victory since the end of white-minority rule in 1994.