Spain’s Carlos Alcaraz saw his hopes of finishing the year as the world number one damaged by a shock opening loss to Roman Safiullin in the Paris Masters.
World number two Alcaraz, who received a bye in the first round, lost 6-3 6-4 to 45th-ranked Safiullin.
The Russian qualifier played superbly throughout and Alcaraz had few answers in a below-par performance.
It leaves Alcaraz with a tough task to overhaul Novak Djokovic at the top of the rankings before the season ends.
Alcaraz, who won Wimbledon in July for the second Grand Slam title of his career, pulled out of Basel last week with injury and was playing for the first time since losing in the Shanghai last 16 on 11 October.
The 20-year-old moved a break up in each set, but Safiullin recovered – returning well to put Alcaraz’s second serve under pressure and also sharp when he came forward from the baseline – to earn the biggest win of his career by ranking.
“Against Carlos, you have to lift up the level and I managed to do this,” said 26-year-old Safiullin, who goes on to play fellow Russian Karen Khachanov in the last 16.
“From Carlos, it was not the best performance but I’m happy to win – even if he is not in the best shape, it is still tough to win. I’m glad I did it.
“Everything – tactical, physical, the mental part – is very important against him. To stay focused from beginning to the end is key.”
Earlier on Tuesday, Djokovic marked his return to match action with a straight-set victory alongside fellow Serb Miomir Kecmanovic in the doubles.
The 36-year-old world number one, who has not played since winning the US Open on 10 September, teamed up with Kecmanovic on Tuesday to win 6-4 6-2 against Ecuador’s Gonzalo Escobar and Kazakhstan’s Aleksandr Nedovyesov.
Djokovic starts his bid for a record-extending seventh title in the singles when he plays Argentina’s Tomas Martin Etcheverry on Wednesday.
American world number 10 Taylor Fritz withdrew before his second-round match with an abdominal injury, damaging his hopes of qualifying the season-ending ATP Finals in Turin next month.
“Devastated to have to pull out with so much on the line,” Fritz said.