There's an unexpected winner in Black Friday's online shopping boom

Mobile ordering has become Black Friday's secret weapon.

By Friday morning, mobile ordering had already proven its dominance for Walmart, Target, and Amazon. Many Black Friday sales started online on Thanksgiving Day, or even earlier in the week.

Target saw a record-breaking day of online sales on Thanksgiving that was due in large part to mobile ordering, with 60% of orders made via smartphone.

On Friday morning, Walmart announced that 70% of traffic to the retailer's website during its Black Friday promotion so far had been from mobile devices. Walmart has made major investments in its app recently, and the retailer decided to offer certain deals exclusively via app for the first time ever this year.

Amazon's mobile ordering on Thanksgiving Day exceeded both Thanksgiving and Cyber Monday's mobile sales in 2015, reported The Street's Brian Sozzi.

"Mobile sales have totaled an incredible $449 million [from midnight to 5 p.m. on Thanksgiving Day], an increase of 58% from this same time last year," Adobe Digital Insights' principal analyst Tamara Gaffney said in a statement Thursday evening. "Mobile has become a standard in the American household, and Thanksgiving has become the day where consumers 'shop on the sly,' as nearly a quarter of people we surveyed said they use mobile devices so they aren't obvious to friends and family at the dinner table."

This year, retailers are both emphasizing online sales the entire week of Thanksgiving and starting Cyber Monday sales earlier than ever. Walmart is stretching the day of online sales into a week-long event, called cyber week, starting on Black Friday. Target is starting Cyber Monday a day early, on Sunday, as well as offering a 15%-off deal in stores as well as online for the first time.

Disclosure: Jeff Bezos is an investor in Business Insider through hispersonal investment company Bezos Expeditions.