World number one Novak Djokovic suffered a shock defeat by Italy's Lorenzo Sonego in the quarter-finals of the Vienna Open.

Djokovic looked out of sorts in the opening set as Sonego took control.

The Italian kept up the intensity in the second set, a double break putting him 4-1 up, and he held his nerve for a famous win.

Sonego lost to Aljaz Bedene in the final round of qualifying but earned a spot in the main tournament after the withdrawal of Diego Schwartzman.

He is yet to drop a set on his way to the semi-finals, where he will face either Britain's Dan Evans or Grigor Dimitrov of Bulgaria.

"It's the best victory of my life. Novak is the best in the world," he said.

"Today I played so, so good. It's unbelievable. It's amazing."

It is the first time Djokovic has ever been beaten by a lucky loser.

It also means he will have to wait a little longer before he completes the formality of equalling Pete Sampras' all-time record of ending the season top of the rankings for a sixth time.

The other semi-final will feature South African Kevin Anderson against fifth-seeded Russian Andrey Rublev.

Anderson beat Russian fourth seed Daniil Medvedev 6-4 7-6 (7-5) while Rubev defeated out-of-form second seed Dominic Thiem 7-6 (7-5) 6-2.