Spain's Carlos Alcaraz, who had aspirations of ending the year as the world number one, suffered a setback with a surprising defeat to Roman Safiullin in the Paris Masters.

Ranked second in the world, Alcaraz, who had a bye in the first round, was defeated 6-3, 6-4 by the 45th-ranked Safiullin. The Russian qualifier displayed exceptional skill throughout the match, and Alcaraz struggled to find effective responses in what was a subpar performance.

This loss presents Alcaraz with a formidable challenge in his bid to surpass Novak Djokovic and claim the top spot in the rankings before the season concludes.

Having secured his second Grand Slam title at Wimbledon in July, Alcaraz withdrew from Basel last week due to an injury.

Returning to competition for the first time since his exit in the Shanghai last 16 on October 11, the 20-year-old managed to secure a break in each set.

However, Safiullin made an impressive recovery, displaying strong returns to put pressure on Alcaraz's second serve. He also demonstrated precision when advancing from the baseline, ultimately securing the most significant victory of his career based on rankings.

“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 made a successful return to the court by securing a straight-set victory alongside fellow Serb Miomir Kecmanovic in the doubles.

The 36-year-old reigning world number one, who had not competed in an ATP tournament since clinching the US Open title on September 10, joined forces with Kecmanovic to triumph with a scoreline of 6-4, 6-2 against Ecuador’s Gonzalo Escobar and Kazakhstan’s Aleksandr Nedovyesov.

Djokovic embarks on his quest for a record-extending seventh singles title as he faces Argentina’s Tomas Martin Etcheverry in the upcoming match on Wednesday.

American ninth seed Taylor Fritz withdrew before his second-round match with an abdominal injury, damaging his hopes of qualifying for the season-ending ATP Finals next month.

“Devastated to have to pull out with so much on the line,” Fritz said.

Germany’s Alexander Zverev and Poland’s Hubert Hurkacz, who are both still hoping to qualify for the Turin event, earned first-round wins in Paris on Tuesday.

Seeded 10th, Zverev staged a comeback after dropping the first set to defeat Hungary’s Marton Fucsovics with a scoreline of 4-6, 7-5, 6-4. Meanwhile, 11th seed Hurkacz secured a hard-fought win against American Sebastian Korda, clinching the match with a score of 6-3, 6-7 (6-8), 6-3.

In another match, Russian fifth seed Andrey Rublev emerged victorious with a 6-4, 6-3 triumph over Japanese qualifier Yoshihito Nishioka during the night session.