Novak Djokovic was stunned by Reilly Opelka in the quarter-finals at the Brisbane International.

Djokovic had been in fine form at the tournament this week, as he prepares for his Australian Open campaign.

Yet the Serbian's hopes of heading to Melbourne with a 100th ATP Tour-level title under his belt were dashed by Opelka, who won 7-6 (8-6) 6-3 on Friday.

The American, ranked at 293rd in the world ahead of the tournament, had never gone up against Djokovic previously, but showed no signs of fear as he forced a first-set tie-break, before breaking twice to take the lead.

Opelka maintained that momentum heading into the second set, and after claiming the decisive break to go 3-1 up, he sealed the win at the first time of asking.

"I just focused on hitting my spots," Opelka said. "If I hit them my spots, it is hard to get it back, but if anyone can it is him. I was ready for that if it happened.

"He is the greatest tennis player the sport has ever seen.

"It is difficult being in Novak's position because he can scout me all day long but the reality is we have nothing to lose coming in against him, he is the greatest player ever. So you end up playing more free and take more risks. If you play your normal level or even a little above your normal level he will win."

Opelka will face Giovanni Mpetshi Perricard in the semi-finals.

Data Debrief: Opelka makes history

Opelka is now up to 207 in the live ATP rankings, but ranked 293rd when he entered the competition, the 27-year-old is the lowest-ever ranked semi-finalist at the Brisbane International.

He is now 8-12 against players ranked in the top 10, though this will go down as his most memorable win to date.