After qualifying a fine ninth, Alonso was in the hunt for a top-10 finish when his race ended after 32 laps.
Suggested to the 39-year-old Spaniard that he appeared to be operating close to his best across the weekend, the Spaniard honestly responded: “I would probably disagree with that.
"I was not at my 100 per cent. It was nice, we had some good battles. Some of them were happy endings, some of them were not because we had been overtaken but I need to find more pace from myself, find more confidence.
“I need to extract more from the car, from the brakes, execute the start better. It was not too bad but I think there is a lot of room to improve from my side."
In his first qualifying session on his F1 return, Alonso eclipsed team-mate Esteban Ocon by reaching the top-10 shoot-out after the Frenchman had been eliminated in Q1.
Referencing specific aspects of his performance that Alonso felt could be improved, he added: “Even the pit stops. I think the guys did an amazing job but my position for the second pit stop was not right.
“These are small things and small mistakes that I keep doing from testing and in this first race. Hopefully, by race two, race three I can perform better personally.”
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