The latest Kantar smartphone report had many interesting tidbits about Windows Phone and iOS market share trends, but perhaps the biggest bombshell was buried in the section about U.S. mobile carriers. T-Mobile’s share of U.S. smartphone sales has collapsed to 9.5% from 12.7% in just a year. At the same time, Sprint’s share has climbed to 12.3% from 11.0% over the same time period. This means that in 1Q 2012, T-Mobile still held a narrow lead over Sprint when it came to smartphone sales in America; by 1Q 2013, Sprint had surged to lead T-Mobile by nearly three points.

This is a major reversal. It shows that even though Sprint lost branded subscribers in 1Q 2013 and T-Mobile gained some branded subs, one core measure of their competitiveness has tilted decisively in favor of Sprint. After all, nobody really cares if a challenger operator picks up feature phone users — the real carrier battle is waged in the smartphone turf.

Sprint is demonstrating that even while its overall subscriber base continues eroding, it is gaining heft in selling premium phones. This could have some ramifications for the never-ending tourney of jostling for a position to get early shipments of important new models in coming quarters. T-Mobile has had trouble landing hot phones for years; its negotiation power with handset vendors may continue fading now that its smartphone share has slipped to single digits.

The Verizon-AT&T duopoly sails on with its share of U.S. smartphone sales now topping 65%. If T-Mobile continues stumbling, Verizon alone could hit a majestic 40% share in 2014.