Novak Djokovic said Thursday he was "concerned" about his physical state, but still dispatched Belgium's Zizou Bergs 6-3, 7-5 to set up a Shanghai Masters semi-final against tournament surprise package Valentin Vacherot.

The 38-year-old Serbian, the highest-ranking player left standing, is getting ever closer to a fifth title in the Chinese financial hub despite struggling throughout the week.

Next in the crosshairs of the 24-time Grand Slam champion is world number 204 Vacherot, a qualifier from Monaco who battled back from a set down to stun Holger Rune 2-6, 7-6 (4), 6-4.

"I'm concerned too," Djokovic said when told his fans were worried about his health, adding there were "issues that I'm trying to address day by day".

"I don't want to talk about the details. I'm still in the tournament. So I prefer just focusing on recovery and trying to give it all."

Vacherot, the lowest-ranked player to make the quarter-finals, said it would be "unreal" to face Djokovic. In stark contrast to the Serb, the 26-year-old said: "What I'm really happy about is my fitness... My body feels amazing."

When he landed in Shanghai, Vacherot was still on the alternate list for the tournament, meaning he wasn't sure he would even get the chance to play in the qualifying rounds.

World number 11 Rune broke in the third and seventh games to win the first set comfortably.

Watched on by his cousin Arthur Rinderknech, who will play in the quarter-finals on Friday, Vacherot took a tense tiebreak in the second set.

Rune looked increasingly rattled and frustration came to a head in the seventh game of the third set when he received a time violation and committed three unforced errors to hand Vacherot a break.

Vacherot served out to secure an unlikely victory, falling to his knees in disbelief. "I just kept fighting harder and harder," said Vacherot. "In the end it was just all about nerves and fitness."