Novak Djokovic joins Rafael Nadal on the sidelines at Qatar Open

Karlovic coped the better with the windy conditions to secure a 6-7 7-6 6-4 victory to reach the semifinals, a surprise even to the 35-year-old who described it as "unbelievable" in his post-match interview.

"When I arrived here, I didn't expect that one. He wasn't able to show his full range because it was windy. I just focused on my game and in the end I was a little luckier."

A tally of 21 took his total number of aces to 67 in three matches and Karlovic also showed his all-court game with 49 winners against the Serbian star.

Djokovic, who had to pull out of the final of the Abu Dhabi exhibition tournament last weekend due to a fever, had shown little ill-effects in beating Dusan Lajovic and Sergiy Stakhovsky for the loss of only six games in earlier matches.

But he but could not find a way past Karlovic, who has now beaten him twice in their three career meetings.

Karlovic, who gained the crucial break of service in the deciding set, hit the 9,000 aces milestone on the ATP Tour on Monday and will next face Spain's David Ferrer for a place in the final.

Djokovic's exit completed a sorry week for the big name attractions in the Qatari capital, with defending champion Rafael Nadal going out in straight sets to German qualifier Michael Berrer in his opener.

It was Nadal's first official match of his latest comeback after being hit by injury and illness at the back end of 2014.

Meanwhile, world number two Roger Federer was also made to struggle against Australian John Millman at the Brisbane International event earlier in the day.

Federer lost the first set of the second round match before rallying to a 4-6 6-4 6-3 victory.

(CNN)