Alcaraz was beaten 4-6 7-6 (7-5) 6-4 by France’s Monfils in a second round match carried over from the previous day because of rain.
The 21-year-old Spaniard, who has won the French Open and Wimbledon titles this year, smashed his racquet after failing to break Monfils, 37, in the third set.
“I felt sometimes that I wanted to break the racquet,” said Alcaraz. “It never happened before, because I could control myself in those situations, in those feelings.
“Most of the time I could control myself and it could go better in the matches or in the situation that I’ve been feeling before.
“Today, I couldn’t control myself, because, as I said, I was feeling that I was not playing any kind of tennis.”
The match resumed in a second set tie-break which Monfils won before claiming the decider for victory.
“I felt like it was the worst match that I ever played in my career,” said Alcaraz.
“I couldn’t play. Honestly, I’ve been practicing really well here in this tournament. The previous days, I was feeling great, hitting the ball clear, moving well. I don’t know what happened.
“I don’t know how I felt like this, but I couldn’t control myself. I couldn’t be better. So this match, it was impossible to win.”
Monfils went on to play Rune later on during the same day and was beaten 3-6 6-3 6-4 by the Dane.
Meanwhile, women’s world number one Iga Swiatek made it into the Cincinnati Open quarter-finals with a comfortable 6-2 6-2 win against Marta Kostyuk.
She will play Mirra Andreeva, who beat Italy’s Wimbledon finalist Jasmine Paolini 3-6 6-3 6-2.
“I’m happy I was solid. I was disciplined with all the things that I wanted to take care of,” said Swiatek.
Aryna Sabalenka secured a 7-5 6-2 victory against Elina Svitolina, while Jessica Pegula won a delayed second-round match against Karolina Muchova 5-7 6-4 6-2 before beating Taylor Townsend 6-2 6-3 in her second match of the day.
