British number one Johanna Konta suffered a shock straight-set defeat by 18-year-old Marketa Vondrousova in the second round at Indian Wells.
World number 11 Konta had four set points in the first set and led 4-2 in the second but lost 7-6 (7-5) 6-4 to her 54th-ranked Czech opponent.
"It was one of the tougher losses I've had in my career," Konta, 26, said.
Konta, who had a bye into the second round in California, has only made one quarter-final in 2018.
"Everybody has these sort of matches and it was my turn to have one of them," Konta said.
"I didn't start off very well and I found it quite difficult to really get any momentum going. It was one of those matches that I had to have, but I couldn't fault my desire to compete out there.
She was erratic on serve against Vondrousova, landing only 51% of her first serves, and was broken in her opening two service games to fall 3-0 behind.
The Briton recovered to lead 6-5, but then failed to take four chances to win the set on the Czech's serve and eventually lost it on a tie-break.
She broke serve in the fifth game of the second set for a 3-2 lead - but then lost four games in a row from 4-2 up to concede the match.
"I still think there's a lot of good things to take from this match in how I competed," Konta said.
"Obviously there are things that I 100% want to be doing better, but I will try not to overanalyse a match like this either."
Konta's only quarter-final this season came in Brisbane in the first tournament of 2018, and she also lost in the second round of the Australian Open in January.
World number one Simona Halep safely progressed to the third round in California with a 6-4 6-4 win over Kristyna Pliskova.
Ninth seed Petra Kvitova had to come from behind to earn a narrow 6-7 (4-7) 7-6 (7-3) 6-4 win over Yulia Putintseva in three hours 18 minutes.
Konta had plenty of opportunities to win this match - but for all the progress she feels she is making off the court, she lacks conviction on it.
After a ragged start, Konta fought back strongly and had four set points to win the first set before playing a scrappy tie-break with a couple of double faults.
In the second set, she led 4-2 but lost momentum after Vondrousova's medical time-out for a toe problem, and lost the last four games.
Konta's serve was not a trusty weapon, and she now heads to Miami to defend her title on the back of just one quarter-final appearance in 10 tournaments.