Zimbabwe's electoral commission has told an all-party meeting that Morgan Tsvangirai won almost 48% in March's presidential election, reports say.
The result would mean the Movement for Democratic Change leader is 2% short of the 50% required to avoid a run-off against President Mugabe, who won 43%.
A source at the meeting told the BBC the figures were tabled at the start of the process to verify the final result.
Earlier, Mr Tsvangirai insisted he had won an outright victory in the poll.
On Wednesday, government sources told reporters that Mr Tsvangirai had beaten the president, but had not won enough of the vote to avoid a second round.
The opposition and human rights groups have accused the authorities of waging a campaign of violence to ensure Mr Mugabe wins any run-off.
The crucial meeting at the Zimbabwe Electoral Commission (ZEC) offices began what officials called the "verification process", in which representatives of the candidates each presented their calculations for the final result.
If the numbers fail to agree, the commission has said the candidates must then go back over the paperwork until they all accept the figures.
"Where we don't agree, we will pull out every relevant document to ensure we have the same figures," ZEC chairman George Chiweshe said beforehand. "At some stage, we have to agree."
According to a BBC source in the meeting, commission officials said Mr Tsvangirai had won 47.8% of the vote, ahead of Robert Mugabe, who took 43.2%.
Although none of the candidates have commented on the meeting and discussions are due to resume on Friday morning, it is almost certain that a dispute is now under way, the BBC's Peter Greste in Johannesburg says.
In an interview with France 24 television earlier, Mr Tsvangirai insisted he had won the 29 March vote outright, and that a second round was unnecessary.
Based on the results his supporters collected from individual polling stations, the MDC has "come up with a result which we feel is credible", he said.
Mr Tsvangirai's electoral agent, Chris Mbanga, is reported to have told the electoral commission on Thursday that the MDC leader had won 50.3%.
Mr Tsvangirai has also accused Mr Mugabe's ruling Zanu-PF party of using the delays to stuff ballot boxes and force the second round run-off, which he says the president plan's to win using a combination of fraud and intimidation.
So, even if the electoral commission orders a full recount, the MDC is unlikely to accept the outcome, our correspondent says.