"I wish to express deepest condolences to the Tsvangirai family on the sad death yesterday evening of Morgan Richard Tsvangirai, the former prime minister of Zimbabwe and leader of the opposition MDC-T party", Emmerson Mnangagwa said in a statement.
The iconic African politician died in Johannesburg, South Africa, where he was reportedly undergoing treatment.
He later contested the first round of the 2008 presidential election as the MDC-T candidate, taking 47.8 per cent of the vote according to official results, placing him ahead of Mugabe, who received 43.2 per cent. He first worked as a plant operator at Trojan Nickel Mine in the northern town of Bindura and entered union politics, becoming the Zimbabwe Congress of Trade Unions secretary-general in 1989.
He was praised for his political courage, but many anxious that he was headstrong and had been ill-equipped to take on Mugabe.
While the MDC won control of Harare, the capital, and other urban areas where Tsvangirai was wildly popular, the security forces helped Mugabe cling to power.
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He boycotted the next round, saying his supporters were being targeted with violence, a move that handed victory to Mugabe. He served as Prime Minister between 2009 and 2013.
The tense relationship between the two was tempered by moments of respect and humor, and the pair once joked about Tsvangirai fearing Mugabe would try to poison him when they started their custom of taking afternoon tea together every Monday.
Mr Mugabe said at the time: "It's not as hostile as before".
He has been in and out of hospital since June 2016, after disclosing that he had colon cancer. "It's all over now - we can shake hands".