Amazon said Thursday that it would shorten its two-day Prime shipping promise to one day.

Walmart fired back late Friday with a snarky tweet hinting that it is planning to follow Amazon's lead.

"One-day free shipping...without a membership fee. Now THAT would be groundbreaking. Stay tuned," the company said in a tweet from its main Twitter account.

Amazon's new one-day shipping promise applies to members of its Prime membership program, which costs $119 annually.

Walmart rolled out free two-day shipping in 2017 without any membership fees, though orders must total a minimum of $35 to qualify.

Read more: Forget one-day shipping: same-day is the 'real battleground' where Amazon, Walmart, and Target, are sparring

Walmart has been working towards one-day shipping for years.

The company said in 2016 that it was capable of shipping packages to 70% of America within one day , but that it hadn't figured out a way to do it profitably, so it wasn't making the service available to customers yet.

Walmart is also building up its same-day shipping muscle.

In conversations between Walmart CEO Doug McMillon and Quo Vadis Capital last October, McMillon characterized same-day shipping as the "final frontier" that "everyone is racing to develop," according to John Zolidis, president of Quo Vadis Capital.

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