Windows RT device was expected to ship upwards of 2 million units.
Microsoft's Surface tablet sold a total of one million units during the fourth quarter of 2012.
The figure was revealed by UBS analyst Brent Thill, which represents a million less from a prior forecast of 2 million.
Within an investors note, the analyst referred to two reasons for his projection: "Surface RT is a consumer device with [fourth-quarter] sales suffering from the difficult iPad compare and narrow distribution."
During early December last year, IHS iSuppli analyst Rhoda Alexander said she expected Surface sales of 1.3 million for the quarter, stating that she "would be surprised to see it much above that."
Thill, meanwhile, has projected a more positive launch for Microsoft's Surface Pro tablet, which delivers a more complete Windows 8 experience when compared to the tablet's current Windows RT variant. The upcoming model is expected to launch on January 26.
Branding the Surface Pro "more promising," Thill believes it's a commendable alternative to Apple's iPad for enterprises users. For Microsoft's fiscal 2013 that ends this June, the analyst estimates sales of 2.5 million units. For fiscal 2014, he foresees sales upwards of 8 million.