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.
Wimbledon champion Garbine Muguruza was also knocked out in three sets by USA’s world number 100 Sachia Vickery.
Konta has won just eight matches since her 2017 Wimbledon semi-final.
The 26-year-old, who had a bye into the second round in California, said it was “one of the toughest losses I’ve had.”
“Everybody has these sort of matches and it was my turn to have one of them.”
World number 100 Vickery stuns Muguruza
Third seed Muguruza, the WTA player of 2017 who reached number one in September, took the opening set 6-2 and went 3-0 ahead in the second in her first meeting with 22-year-old American Vickery.
But the 24-year-old Spaniard had no answer to a stunning Vickery fight-back, which combined precise groundstrokes with some deft touches, and the American claimed 13 of the next 16 games.
Muguruza served a total of five double faults in the match and was defeated in two hours 11 minutes.
It was the Spaniard’s first loss to a player ranked in the 100’s since a 6-3 6-2 defeat on Court One at Wimbledon against Jana Cepelova in 2016.
For crowd favourite Vickery it was a first victory against a top-10 player and she said: “The atmosphere really helped me even though I was feeling it a bit physically. I kept telling myself ‘just do your best’.”
Vickery marked her win with the crossed arms Wakandan symbol from the Marvel Comics film Black Panther, which she admitted to having watch four times.
“That was definitely Wakanda Forever. I’m so obsessed with the movie,” Vickery said. “It’s taking over my life, literally, I’m obsessed.”
Elsewhere, world number one Simona Halep progressed to the third round with a 6-4 6-4 win over Kristyna Pliskova of the Czech Republic, whose identical twin Karolina saw off Romania’s Irina-Camelia Begu 7-6 (7-4) 6-1 in an hour and 32 minutes. .
French Open champion Jelena Ostapenko, who lost her opening matches in both of her last two tournaments, overcame fellow 20-year-old Belinda Bencic 6-4 3-6 6-1 in one hour 51 minutes.
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 a gruelling three hours 18 minutes.
‘Difficult to get any momentum going’
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.
“I didn’t start off very well and I found it quite difficult to really get any momentum going,” she admitted. “It was one of those matches that I had to have, but I couldn’t fault my desire to compete out there.
“I don’t think this overshadows at all the good work that I’m doing.”
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.”
BBC tennis correspondent Russell Fuller
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.