Confirmed. His ankle was bothering him since yesterday:
From Tennis Panorama twitter
@TennisNewsTPN Tennis Panorama News
Kei's ankle was bothering him since yesterday, just wanted to double check with trainer. Decided to finish the match
Kei said in the interview yesterday it took a few games after rolling the ankle to recover. After one day, however, maybe the pain flared up again during the first set.
You know the way he rolled the ankle was a bit similar to the way Juan Carlos hurt his ankle when he was leading Ferrer in the second set. After that, he was never the same in the match.
Could be something to do with the surface. I vaguely remember many players rolling ankles in Shanghai last year or a couple of years ago.
In any case, rest up well, Kei, and good luck in Vienna!