Carlos Alcaraz slipped to a shock defeat on his return from injury on Tuesday at the Paris Masters while Alexander Zverev needed three sets to see off Marton Fucsovics.
Spanish world number two Alcaraz, who was making his return to the court following injuries to his lower back and left foot, had been given a bye in the first round but had no answer to Russian qualifier Roman Safiullin who eased through 6-3, 6-4.
"He didn't surprise me at all because I knew that he has been playing a great level these last few months, beating big guys, reaching finals," said a disconsolate Alcarez.
"I knew that he was going to play a high level."
Safiullin, ranked 45 in the world, cancelled out an early Alcaraz break in the first set and then never looked back as he ran on to claim the opener 6-3.
The same scenario then played out in the second set, with Alcaraz breaking for an early lead only for his opponent to strike back immediately.
Having only announced he would play in the tournament last Wednesday, Alcaraz looked far from his sharpest.
"I just didn't feel well, you know, on the court," he said. "A lot of things to improve, a lot of things to practice.
"I think I didn't move well. In the shots, I think I had a good quality of shots. But physically, in terms of movement, I have to improve a lot."
However, much credit must go to Safiullin, who was relentless in getting over the line for a career-boosting win in what was his first meeting with the Spaniard.
The 26-year-old held his nerve under pressure from the Wimbledon champion to serve out for the win, despite facing several deuce points in the final game.
"Even if he's not in the best shape it's tough to beat him," said Safiullin. "So, I'm really happy that I made it."
- Zverev eyes ATP Finals -
The German 10th seed Zverev also had his difficulties, having to come back from a set down to beat Hungarian Fucsovics 4-6, 7-5, 6-4 in the round of 64 at Bercy Arena.
"It was just one or two points (that made the difference)," said Zverev.
"He was playing incredibly well and tactically I think he was unbelievable.
"His slice was effective as I did not know what to do on this surface here. It was a very difficult match, and I am happy to be through. I found my level and I am happy with that."
Zverev is still seeking to secure his place among the top eight who will go on to the ATP Finals in Turin next month.
After this victory, the 2021 champion sits seventh just behind Stefanos Tsitsipas and 430 points ahead of ninth-placed Hubert Hurkacz.
- Fritz pulls out -
American Taylor Fritz's chances of qualifying for the tour's showpiece event took a hit when he was forced to retire with an abdominal injury.
On Monday, the ninth seed had comfortably beaten Argentine Sebastian Baez 6-1, 6-4.
Hometown favourite Gael Monfils was knocked out in the first round by Argentinian world number 21 Francisco Cerundolo in three sets on Tuesday.
The battling 37-year-old looked tired after losing the second set in a tie-break but rolled back the years with a break of serve right at the start of the third.
However, it was not enough as Cerundolo struck back in the eighth game of the set, before breaking the Frenchman's serve again to take the decider 7-5.
The 11th-seeded Hurkacz saw off Sebastian Korda 6-3, 6-7 (6/8), 6-3 to book his place in the second round alongside Felix Auger-Aliassime, who toppled Jan-Lennard Struff in two sets.
Russian fifth seed Andrey Rublev completed the day's play with a quickfire 6-4, 6-3 victory over Japanese qualifier Yoshihito Nishioka.