Novak Djokovic's hopes of ending the season as world number one for a record seventh time are on hold after Daniil Medvedev battled past French qualifier Hugo Gaston 7-6 (9/7), 6-4 to reach the Paris Master semi-finals on Friday.
Djokovic had eased into the last four earlier on Friday, beating American Taylor Fritz 6-4, 6-3, and would have been confirmed as season-ending world number one had Gaston beaten Medvedev.
Djokovic has already suffered one crushing disappointment at the hands of Medvedev this year, the Russian dashing his dreams of a Grand Slam sweep when he beat him in the US Open Final in September.
That prompted Djokovic to take a seven-week break and he admitted his decision to return this week was down to ensuring he retained the top spot.
"That's actually the biggest reason why I came here, trying to clinch the year-end No. 1," said Djokovic.
"Hopefully I can do it during this week. If not, then I'll have another chance in Turin (ATP Finals November 14-21)."
At one point it looked as if it would be a good Friday for the 20-time Grand Slam champion as Gaston, once again playing well above his ranking of 103, served for the first set.
However, Medvedev broke and then won the tie-break before surging into a 4-0 lead, and although the 21-year-old Frenchman got a break back the defending champion held his nerve to serve out for the match.
Medvedev will play the winner between last year's finalist, Olympic champion Alexander Zverev and Norway's sixth-seeded Casper Ruud.
The match brings together the only two players to have won five titles apiece this year, that feat propelling Ruud to a first ever ATP Finals appearance for a Norwegian player.
Hubert Hurkacz became the eighth and final player to qualify for the ATP Finals after seeing off a stern challenge from Australian journeyman James Duckworth 6-2, 6-7 (4/7), 7-5.
The 24-year-old Pole will be looking to go one better than his compatriot, 2012 finalist Jerzy Janowicz, in Paris.
He faces a mighty big hurdle to even get to the final as he plays Djokovic, who looked in ominously good form against Fritz.
Hurkacz may have more in the locker than Fritz as he has shown in a breakout season, reaching the Wimbledon semi-finals and winning the Miami Masters.
"It is crazy," said Hurkacz referring to his Masters qualification.
"The match was so hard today, knowing that I had to win the match.
"That obviously upped the pressure a little on me too."
Hurkacz had looked well in control as he eased through the first set 6-2.
However, 55th ranked Duckworth levelled the match with a commanding performance in the tie-break -- wagging his fingers at the crowd after taking it 7/4.
It was an equally hard fought third set and Hurkacz needed three match points before taking the honours, the seventh seed raising his arms and then punching the air in delight.
"You dream about it as a kid, and you're, like, when you're growing up you don't know if it's gonna be possible, if you're gonna be good enough," said Hurkacz of reaching the ATP Finals.
"Now achieving this thing, it's very big for me."