Qatar Holding said last week it opposed the terms of the merger which if completed would create a commodities behemoth worth around $87.3 billion) according to the current market capitalisation.
stressed in a statement that "although it continues to support the principle" of the merger, it has determined that it will not support the current terms.
Glencore, which owns nearly 34 percent of its coveted compatriot, is not authorised to vote during Friday's general assembly in the small city of Zug in central Switzerland.
The firm has so far staunchly stood its ground amid the mounting pressure from Xstrata's shareholders and has even begun bracing for the possible failure of the deal.
Glencore chief, Ivan Glasenberg of South Africa, told Dow Jones Newswires last month that the merger was "not a must-do deal".
"We cannot overpay on this asset," he said, adding: "If it doesn't happen, it's not the end of the world, we move forward."
Glasenberg also insisted he had not received any persuasive arguments for why his company's offer needed to be raised.
"No one has given me any substantive figures" showing that exchanging 3.25 Glencore shares for each of Xstrata's was a reasonable deal.
The outcome of Friday's vote is basically already clear, experts say.
"Glencore's takeover probably will fail unless Glasenberg raises the offer in time," the CIMB investment bank said in a note.
"We think there is a good chance that Glencore could maintain its 2.8 bid going into this Friday's vote (likely) resulting in a vote down of existing
terms," RBC Capital Markets agreed.
Xstrata's and Glencore's general assemblies were already postponed once at the beginning of July due to Qatar Holding's demand for better merger terms, as well as grumbling over bonuses being handed out to 73 Xstrata top executives, including chief executive Mick Davis.
If the merger does fall through, it will not be the end of the world for either of the companies though, analysts say.
In the first half of the year, Glencore posted a slightly lower net profit at $2.3 billion, but said it remained in a solid position to grow its revenue.
Xstrata meanwhile saw its net profit slump 33 percent in the first half of the year to $1.9 billion, but is benefiting from good organic growth, according RBC.