World No 1 Carlos Alcaraz said he would "take lessons" from his shock first-round exit at the Astana Open on Tuesday after losing 7-5, 6-3 to Belgium's David Goffin.
US Open champion Alcaraz suffered his first straight-sets defeat since last November as Goffin made the most of his second chance as a lucky loser.
"He played better than me, really, really aggressive," said Alcaraz.
"I couldn't handle that pressure that he was pushing on me and of course it's something that I have to learn and take lessons from this match."
Alcaraz, 19, was playing his first ATP tournament since his Grand Slam triumph in New York that saw him become the youngest ever world No 1.
He then flew immediately to Valencia to play in the Davis Cup, winning one of his two singles rubbers as Spain claimed a place in the quarterfinals.
"Coming back to competition is never easy," said Alcaraz.
"He (Goffin) played two matches here on this court. It's not easy to get used to, it's really, really slow. It was really tough on me, for my confidence.
"I tried to get used to this court so quickly, but I couldn't and he was better than me in this match."
Goffin only earned a spot in the main draw after the withdrawal of Danish teenager Holger Rune. He plays Frenchman Adrian Mannarino in the last 16.
"I always believe that I have the level to cause some trouble against those guys," said Goffin, a former top-10 player now ranked 66th.
"When you play against the world No 1 on a big stage, big crowd, the fire inside gives you so much power to play your best tennis because you don't have any choice.
"You have to fight and give your best and that was the case today. I always believed I had it and I am happy you can still it there."
It was 31-year-old Goffin's third win against a top-ranked player, having beaten Rafael Nadal in 2017 and 2020.
Alcaraz was last beaten in straight sets by France's Hugo Gaston at last year's Paris Masters. He had taken at least a set in 62 matches this season.
Daniil Medvedev powered to a 6-3, 6-1 victory over Albert Ramos-Vinolas as he attempts to nail down a place at the season-ending ATP Finals in Turin.
Third seed Stefanos Tsitsipas eased past Mikhail Kukushkin 6-3, 6-4.
The Greek is hunting a third title of the season. He reached 50 tour-level wins in 2022 after breaking Kukushkin once in each set.
"It was a great match from my side," said Tsitsipas, who goes on to face Italian qualifier Luca Nardi for a quarterfinal place.
Karen Khachanov won on his first outing since reaching the US Open semifinals, defeating American Maxime Cressy 6-4, 7-6 (7/3).
Novak Djokovic is due to face Cristian Garin on Wednesday after winning the 89th title of his career in Tel Aviv last weekend.