Andy Murray admitted he was low on confidence and concerned about his form after a defeat by Milos Raonic that he described as "not good enough".
The fourth-round loss in Indian Wells was particularly disappointing for the Wimbledon champion because for much of the match he looked to have found his form.
Murray put his struggles against Jiri Vesely on Monday behind him to win the first set against big-serving Raonic.
The 10th seed hit back to level matters but Murray's worries stem from the way he played in the third, dropping his serve twice in a row having led 2-1 to go down 4-6 7-5 6-3.
The Scot said: "The third set, that's concerning for me. The rest of the matches I have played since I have come back, I haven't been particularly concerned about them.
"Australia was good, Davis Cup was good, Acapulco was good. Today was fine until I went ahead in the third set, and then I was poor. It wasn't good enough.
"To get broken two consecutive times in that situation isn't good enough. I played poor tennis at that stage. I didn't make enough balls, missed easy shots.
"But like really easy shots, not like ones that are deep balls where you're just trying to get it back into play. There were some where he was standing at the net and I just had to hit it to the other side."
Asked if had an explanation for his collapse, he said: " I don't know.
"It wasn't like I was doing it the whole match. I was playing decent. Both of us didn't exactly have loads of opportunities on the other's serve until that middle part of the third set, so that's probably down to confidence.
"Because there is no other reason for missing shots like that, basic tennis shots that I would hope to make 99 times out of 100."
The BNP Paribas Open was Murray's fifth tournament since back surgery, and he arrived in the Californian desert confident that he was heading in the right direction.
He reached the semi-finals in Acapulco two weeks ago, losing a close match against Grigor Dimitrov, and insisted that physically he is feeling good.
He said: "It shouldn't really be down to not playing enough matches. I feel like I've played enough.
"Right now I'm not pulling up after matches stiff or sore or anything like that. So I think I'm match fit, match tight. Maybe I'm making some bad decisions in those situations.
"But I'll need to have a think about it and have a look at what happened. All of the shots kind of deserted me a bit."
Murray now heads to his second home of Miami for next week's Masters tournament, and the consolation will be that, unlike Indian Wells, Crandon Park is a venue where he has had a lot of success.
The Scot has won only eight matches in his last five appearances at the BNP Paribas Open but initially looked to be playing a very smart match against a difficult opponent.
Raonic sent down a 140mph ace on the first point but Murray withstood the barrage from the other end and capitalised when the Canadian's level dropped, winning 10 straight points to take the set.
He looked the more likely winner of the second but this time it was Raonic's turn to hang in there, and Murray showed his frustration as half chances slipped away.
Raonic sensed his opportunity late in the set and played a fine game to break Murray's serve for the first time and level the match.
The momentum looked to be with the fifth seed when Raonic played a sloppy game to be broken but Murray promptly gave it straight back and then dropped serve again to trail 4-2.
He saved one match point on his own serve but a comeback never looked on the cards and Raonic sealed victory with a forehand winner to set up a quarter-final against Alexandr Dolgopolov.