Microsoft has raised its Surface Pro 2-in-1s' monthly shipments to 500,000 units for the fourth quarter, up 60% from 300,000 units originally to prepare for the year-end holiday sales, according to sources from the related upstream supply chain.

Surface Pro 4's manufacturer Pegatron Technology is expected to benefit from the increasing orders. The Surface Pro 4 features a 12.3-inch display and Intel's Skylake processor to boost performance by 30% compared to the Surface Pro 3 and 50% compared to the MacBook Air, the sources noted.

Microsoft sold about 1.5 million Surface Pros in the first half, according to Gartner, up significantly from the first half of 2014. The sources expect the Surface series' overall shipments to reach 4-5 million units in 2015.

Currently, Microsoft's Surface 2-in-1 shipments only account for around 1% of overall Wintel PC shipments because of its above US$1,500 price level, but PC brand vendors still find the product line a threat since the PC market is gradually shrinking and vendors have been looking to expand their high-end product lines to boost profits.

With China-based smartphone brand vendors Xiaomi and Huawei planning to expand into the notebook industry to compete for the entry-level and mid-range markets, and Microsoft and Apple threatening the high-end market, PC brand vendors are expected to face more difficulties to achieve growth.