The Israeli needed attention on what appeared to be a groin injury and ended up going off court with the trainer at the end of the first set to receive treatment.
The 10-minute interruption failed to throw Dimitrov off stride and once he was up 6-1 6-1, Sela indicated he could no longer continue.
The Bulgarian 13th seed will face either title favourite and seven-times champion Roger Federer or Germany's Mischa Zverev.
(Reporting by Pritha Sarkar, editing by Clare Lovell)
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