Novak Djokovic's hopes of making a record 19th appearance at the ATP Finals have been dented by a surprise defeat against Jan-Lennard Struff at the Swiss Indoors in Basel.

Playing in the tournament for the first time in 15 years, the 39-year-old lost 6-7 (4-7) 7-5 7-6 (7-5) - the first time he has been beaten by Struff in nine meetings.

"I knew I needed to go for it. There was only one way to play it. So, I was very, very happy," said the world number 27.

"I knew I had to play my best tennis to have a chance. The last eight times he beat me very, very easily. So, I'm very, very happy and pleased with the performance."

The year's top-eight performers qualify for the ATP event in Turin and this result leaves Djokovic - the men's game's record 24-time Grand Slam winner - 11th in the standings.

Djokovic admitted he expected to feel "a little rusty" after his win against Frenchman Ugo Humbert on Wednesday, but now faces a fight to make the finals as he is outside the top 10 for the first time since 2018.