Rafael Nadal lost 6-2 7-5 in the quarter-finals of the Italian Open on Saturday night to Diego Schwartzman.
The Spaniard, ranked at number two in the world, was almost immediately on the back foot, and slumped to a 6-2 first set loss.
In the second and final set, eighth-ranked Argentine Schwartzman allowed his opponent back into the match with at 5-5 before taking two consecutive games for the win.
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"We can find excuses, but I didn't play well enough. Then we have to think internally, 'Why? How I can fix it?'" the 34-year-old Spaniard said. "Now is not the moment to find excuses. It's just to accept that I didn't play well enough."
"It was not my night. He played a great match, I think, and I didn't. These things can happen. After such a long time without competing, I played two good matches, and now I played a bad one against a good opponent.
"That's how it is. I just want to congratulate Diego and I'm going to keep working."
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Nadal, who skipped the US Open in New York due to COVID-19 concerns, said he would regroup before heading to Paris, where he will bid for a record-extending 13th title.
"It's a completely special and unpredictable year. I probably will go back home and then let's see what's going on," the 19-time Grand Slam champion said.
"I did my job here. I did a couple things well and other things bad. I'm going to keep working and keep practising with the right attitude and try to give myself a chance to be ready."
Earlier in the day Novak Djokovic needed three sets to defeat Domink Koepfer to reach the semi-finals for the Roland Garros warm-up event.
Additional reporting from Reuters