World number one Novak Djokovic is out of the Vienna Open after a shock 6-2 6-1 quarter-final defeat to unfancied Lorenzo Sonego.

Ranked 42 in the world, Sonego had failed to win a single set in his three previous meetings with players inside the top 10.

But the 25-year-old Italian produced a scintillating performance against the top seed as he became the first lucky loser to beat Djokovic.

"It's unbelievable. It's amazing," Sonego said in an interview on court after wrapping up the win in 68 minutes.

"For sure, it's the best victory of my life. Novak is the best in the world. I played so, so good."

Djokovic's surprise exit at the Wiener Stadthalle meant the Serbian missed the opportunity to guarantee top spot in the year-end ATP Rankings this week.

The 33-year-old is still poised, however, to tie Pete Sampras' record of six year-end number one finishes unless Rafael Nadal takes up an unlikely wildcard for next month's Sofia Open, with reports indicating the Spaniard has already rejected such an opportunity.