Election officials in Afghanistan are expected soon to announce whether Karzai is the outright winner of August's disputed election or must face a second vote against rival candidate Abdullah.
The UN-backed Electoral Complaints Commission (ECC) may unveil its findings of a probe into fraud allegations on Sunday or Monday.
If enough votes are disqualified, the final result may push Karzai's share of the vote below 50 percent, the incumbent would then face Abdullah in a second round - barring possible legal steps to invalidate the decision or his rival's decision to withdraw.
But the expectation is that there will be a runoff vote.
"Even some of Karzai's closest aides are speculating that there will be a second round of voting. For example, the Afghan ambassador to the United States - Saeed Jawar - says he believes it is likely", Al Jazeera correspondent James Bays said.
And Aleem Siddique, spokesman for the UN mission which appointed three of the ECC's five members, said preparations for a possible run-off were already underway, including measures designed to eliminate any risk of repeated fraud.
"Where required, staff will be replaced [at polling stations]," he said. "Polling stations will not open where security could lead to attempted fraud."
On Saturday, US Secretary of State Hillary Clinton told CNN in an interview that she believed Karzai would win a second vote. "It is likely that they will find that President Karzai got very close to the 50+1 percent," she said.
"I think one can conclude that the likelihood of him winning a second round is probably pretty high."
However, the chairman of the independent election commission - Azizullah Ludin - has not yet received a final report from the Election Complaints Committee on voting fraud.
Ludin said he will need at least a couple of days to consider what its final conclusions should be before any announcement might be made.
The August 20 election has left Afghanistan in a state of political uncertainty at a time when the United States is deciding whether to send more troops there to fight a resurgent Taliban.
Karzai won 54.6 percent of the vote, according to preliminary figures. More than 250,000 votes will have to be thrown out from his tally for it to fall below 50 percent.