The setback was a third straight loss for the 30-year-old Serb, who is trying to make a comeback after an elbow injury sidelined him for the final six months of last year.
A dejected Novak Djokovic went searching for answers after losing his opening match at the Miami Open on Friday.
"I'm trying, but it's not working. That's all. That's all it is,” the former world number one told reporters after a 6-3, 6-4 defeat by Frenchman Benoit Paire.
The setback was a third straight loss for the 30-year-old Serb, who is trying to make a comeback after an elbow injury sidelined him for the final six months of last year.
It also snapped a 16-match winning streak in Miami for the six-time tournament winner.
“I'm not feeling great when I'm playing this way," said Djokovic, who was seeded ninth in Miami.
"Of course, I want to be able to play as well as I want to play. Just it's impossible at the moment. That's all. I lost to a better player."
Serving in the first set, Djokovic had an opportunity to reach 5-5, but lost five consecutive points and the set.
He appeared to be fighting back in the second set, winning eight of nine points to recover a break, but was broken to love in the final game of the contest.
“I felt I started the match well, first six games, then I just ran out of gas,” Djokovic said. “He was serving well. I just wasn't able to break him down. He was just coming up with the good shots at the right time. It happened very fast.”
Where the future will take him, the Serb, playing in only his third tournament since Wimbledon last year, is not certain.
“I know that you can't be the person that you were yesterday, and the player (you once were)," Djokovic said.
“The circumstances that I was in the last two years were very challenging. But I'm not the only one that goes through that. I mean, there are tougher injuries that players go through.
"I don't want to sit here and whine about my last couple of years.”
