Zverev bristled at Navratilova’s criticism but the first two sets here were a horror show for the German.
The scoreline could have been worse given Zverev trailed 5-1 in the first set, 5-0 in the second – a set where he looked completely lost and hit 22 unforced errors in just eight games.
Zverev had been a set and a break down against Borna Coric in the last eight before turning things around, and he at last began to play more freely at the beginning of the third set.
With Carreno Busta, who played a four-hour marathon against Denis Shapovalov in the quarter-finals, starting to lose some physical sharpness, it became apparent the match still had some way to run.
Zverev made only three unforced errors in winning the third set and, although his new-found grip on the match remained tenuous, he won the fourth as well to force a decider.
Carreno Busta took a medical timeout for treatment to his pelvic area but, if he had hoped that would help him halt Zverev’s momentum, he was to be disappointed.
The Spaniard was now looking very weary, and a resurgent Zverev broke serve again to start the decider before clinching victory on his second match point.