The world number four and top seed was beaten 7-5 3-6 6-1 by the Russian qualifier on Wednesday in what was a major upset at the tournament.
"I couldn't seem to hurt him with my shots," Ferrer is quoted as saying by Sky Sports News. "It was a strange match mentally and physically.
"This was a match to forget - probably one of the worst I've played in recent years."
Tursunov's reward for beating Ferrer is a third-round encounter with Thomas Bellucci.