Ewan (Lotto Soudal) won the bunch dash into Sisteron on Tuesday morning (AEST), taking stage honours after a drag race between the sprinters.
The Australian emerged late from the chaos of the sprint, flying down the barriers and passing Sam Bennett (Deceuninck-QuickStep) in the final metres to claim the victory.
McEwen, a triple winner of the Tour de France's green jersey, analysed his countryman's sensational win via the SBS touchscreen.
"Ewan was in 17th position with 300 metres to go," McEwen said.
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"It was a really tight squeeze but what I loved about Caleb Ewan's sprint was that he fully committed to that gap.
"He thought to himself 'if it's going to close, I'm going to hit it as hard as I can, it's the only possible way I'll go through and the only way I'm going to get the victory on this stage'.
"It was absolutely brilliant."
