Jannik Sinner saw his streak of 37 consecutive sets won at Masters 1000 level ended by Tomas Machac as the Italian reached the quarter-finals of the Monte-Carlo Masters with a 6-1 6-7(3) 6-3 win.
Last month Sinner became the first man to complete the Sunshine Double without dropping a set, claiming titles at Indian Wells and the Miami Open, and had also won the Paris Masters title last year.
Despite the setback, Sinner recovered to seal victory over his Czech opponent to continue his charge to dethrone the current world No. 1 Carlos Alcaraz.
Not every day is the same. I was struggling a little bit, I was a bit tired," Sinner said.
"I felt great before the match. In the second set I struggled a bit to find the right energy, but this can happen.
"I tried to push myself through, which I have done. Even when you don't feel your best, you try to find ways and that was the case today and I am happy. The main priority is to recover."
Sinner faces Felix Auger-Aliassime in the quarter-finals after the sixth seed progressed when Casper Ruud was forced to retire with the Canadian leading 7-5 2-2.
Alcaraz also dropped a set before taking a 6-1 4-6 6-3 win over Argentina's Tomas Martin Etcheverry to continue his title defence.
The Spaniard fell into an unexpected battle in the second set when he made 23 unforced errors to allow Etcheverry, the reigning Rio ATP 500 champion, to storm to a set win and force a third.
But the pressure on Alcaraz isn't just from his opponents on the court, as a title run for Sinner could mean he bumps himself back up to the world No.1 spot.
"I was just feeling the ball really well in the first set, hitting the ball like 10 out of 10, I would say," Alcaraz said.
"So and then in the second set, I started playing well as well. I just had chances to break his serve in the second game, but I just didn't take it.
"And as I say every time, when you don't take the opportunities in this kind of level, you have to run back.
"So I would say he gained a little bit of confidence after that, playing more aggressively, hitting the ball better, and not making that many mistakes.
"After that, I would say the conditions were a little bit slower, which became a little bit tougher, but I would say that was a great quality fight at the end. "
Alcaraz will play Alexander Bublik in the quarter-finals, the first time the pair will play one another in an official tournament, after the Kazakhstani took a 6-2 7-5 win over Jiri Lehecka in 75 minutes.
Elsewhere, 19-year-old Joao Fonseca eased past Matteo Berrettini 6-3 6-2 to reach the quarter-finals of a Masters 1000 event for the first time.
While third seed Alexander Zverev defeated Zizou Bergs 6-2 7-5 to reach the tournament's last ight for the first time since 2022.
Australia's Alex De Minaur took a tight 7-5 7-6 (7-4) win over qualifier Alexander Blockx, who had celebrated his 21st birthday the day before, after early exits at the Miami Open and Indian Wells.
In the final match of the day, Valentin Vacherot made history as the first Monegasque in the Open Era to make the top eight in Monaco, following his 6-7(4) 6-3 6-4 win over Hubert Hurkacz.
Vacherot's victory on home soil comes just eight months after his breakthrough where he became the lowest-ranked Masters 1000 champion since the series' inception in 1990 when won in Shanghai as the world No.204.
The second and third seeds, Ekaterina Alexandrova and Liudmila Samsonova, were knocked out of the Linz WTA 500 tournament on Thursday.
Samsonova was beaten by 18-year-old Lilli Tagger, a local wildcard, who is through to her first clay quarter-final against Anastasia Potapova.
While 34-year-old Karolina Pliskova of Czechia defeated Alexandrova to ease through to the final eight.
Anastasia Potapova, Donna Vekic, Lina Gjorcheska, Marina Bassols Ribera, Elvina Kalieva and Lisa Pigato all won their matches to complete the top eight.
Three-time Grand Slam quarter-finalist Holger Rune will return to action at next month's Hamburg Open after having Achilles surgery in October.
Rune suffered the season-ending Achilles tendon rupture in the Stockholm Open semi-finals when he was up a set against Ugo Humbert.
Days later, he returned to the top 10 in the world rankings despite losing out on a second title of the year.
"The hard work starts in Hamburg. I can't wait to be back on clay at the Bitpanda Hamburg Open and to finally experience the atmosphere on site again after such a long break," the 22-year-old Dane said in a statement.
The Hamburg Open starts on May 16, with Germany's world No 3 Alexander Zverev also taking part.
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