World number one Novak Djokovic was buried by 21 aces as big-serving Croatian Ivo Karlovic clinched a 6-7 (2/7), 7-6 (8/6), 6-4 victory to reach the Qatar Open semi-finals on Thursday. Top seed Djokovic joined second seeded Rafael Nadal on the sidelines in Doha after the Spanish star lost in the first round. Karlovic, the 35-year-old world number 27, claimed his second career win over the world number one having also triumphed at the Madrid Masters in 2008. He will face Spanish fourth seed David Ferrer for a place in the final.