WATCH: David Goffin forced to retire after suffering freak injury in Rotterdam

The Belgian fourth seed had lost the first set 6-3, but was a game up in the second set when he attempted to play a backhand across court from a Dimitrov passing shot.

Somehow the ball came off his racket before popping up to connect with his eye at close range.

He immediately dropped to the ground covering his face from what appeared to be an innocuous incident on TV replays.

Goffin was forced to take a time-out on his seat with his head covered by a towel, but the problem was serious enough for him to retire with second seed Dimitrov waiting to continue his service game as the sympathetic crowd applauded the players off the court.

New world number one Roger Federer or Andreas Seppi will contest the second semi-final at the Dutch indoor tournament.