World number one 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 number one.
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 quarter-finals.
"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.