World number four Sinner is guaranteed at least second spot in the Green Group thanks to Hurkacz, who played his only match of the season-ending tournament as a replacement for injured Stefanos Tsitsipas, claiming a set against Djokovic.

The Serbian world number one is now sweating on his semi-final place despite a second win from three matches, as his qualification hinges on the result of Sinner's final match with Holger Rune later on Thursday.

Sinner could deliberately lose against Rune and knock Djokovic out while still going through, denying the 24-time Grand Slam champion the chance of winning a record-breaking seventh Finals.

That would be a disappointing end to what has been an age-defying 2023 from the 36-year-old who won three Grand Slams and took his 1000 Masters crowns to 40, another record.

However, a win over fellow young gun Rune would give Sinner first place in the group and likely mean missing on-form Red Group leader Daniil Medvedev in the semis.

Djokovic now has to hope Sinner does beat Rune, who is more rested than the Italian after only having to play three games in his match with Tsitsipas on Tuesday before the Greek dropped out of the tournament injured.

He took control at the end of the first set when he rattled off nine points in a row to take a 6-0 lead in the tie-break and then seal the set.

However, he threw away his serve to love in game five of the second set and two games later looked on in disbelief as Hurkacz rammed home four consecutive aces from 15-40 down to go to within one game of drawing level.

Hurkacz easily saw out the set, drawing warm applause from the home crowd, and with the third set almost irrelevant the Pole collapsed to a visibly annoyed Djokovic.