DOHA (AFP) - World No. 1 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.

Djokovic appeared hampered by a stiff wind, which did not prevent Karlovic from hammering down the aces to take his total in three matches to 67 - but made it hard for the favourite to control his ground strokes.

Djokovic was nevertheless twice within two points of winning the match in the second set tie-breaker, before dropping serve in the seventh game of the final set. He never looked likely to make a break back.

"I was feeling good despite the wind," said Karlovic. "I know he wasn't able to show his full range. I was relaxed and didn't feel I had to do anything extraordinary. I just had to focus on my game."

Karlovic, the 35-year-old world No. 27, claimed his second career win over the world number one having also triumphed at the Madrid Masters in 2008.

The 210cm Croat, the tallest and oldest player on the main tour, will face Spanish fourth seed David Ferrer for a place in the final.