Amazon Set to Grab Market Share From Wal-Mart: William Blair Inc (AMZN.O) is well-positioned to grab market share from Wal-Mart Stores Inc (WMT.N) and Target Corp (TGT.N) given its competitive pricing and wide assortment, analysts at William Blair & Co said, and upgraded Amazon to "outperform."

The analysts downgraded Target and Wal-Mart to "market perform," and raised doubts over Wal-Mart's ability to sustain earnings growth in its U.S. operations, due to potential consumer pressures from rising gas prices and reduced fiscal stimulus.

In addition, the margins and returns of new Wal-Mart Express stores can drag on results, said the analysts, who had previously rated Wal-Mart and Target "outperform."

"'s pricing to the consumer is superior to and, as we found the retailers charge higher shipping fees on identical items," the analysts wrote in a note to clients.

The proliferation of smartphones and mobile commerce could also vastly increase price transparency, which will enhance the growth opportunity for, they said. shares were up about 2 percent at $168 in premarket trade on Thursday, after closing at $165.32 on Nasdaq on Wednesday.

Shares of Wal-Mart closed at $51.64 on Wednesday on the New York Stock Exchange, while those of Target closed at $50.24.