The 22-year-old, who had achieved her best result at a major by reaching the last eight at Melbourne Park, lost 6-3 6-0 to Dominika Cibulkova for a place in the last four.
"I just couldn't play today," said Halep. "I had emotions, big emotions, and I couldn't manage this. Before the match I was very nervous and I didn't feel the ball at all. I couldn't move my body and I couldn't play."