Belinda Bencic made a painfully sharp exit from the Qatar Open as she was beaten 6-4 3-6 6-3 by rising star Clara Tauson.
The Olympic champion was the highest-profile casualty on day one, losing to a player making her main-draw debut in a WTA 1000 event.
Only the second Danish player to appear in Doha, after former world number one Caroline Wozniacki, the 19-year-old Tauson came from 4-2 down to take the opening set, before Bencic levelled up.
But Tauson was not to be denied. The world number 33 gained a crucial break in game four of the decider, before winning 12 of the last 13 points on her serve to set up a second-round clash with third seed Paula Badosa.
"It was a very tough match," said Tauson, who hit seven aces. "I know Belinda is playing very well, so I had to play my best today, and I did sometimes.
"I have my confidence, but every match is really tough at this level, so I'm doing my best."
Two-time champion Victoria Azarenka survived a scare to scrape past Kazakhstan's Yulia Putintseva 5-7 6-2 7-5.
The 12th seed, who is aiming to become the first player to lift this trophy three times, recovered from two breaks down in the decider before rescuing match point at 5-4 down and eventually scrambling over the line.
Amanda Anisimova, who defeated Bencic and Naomi Osaka at the Australian Open, built on her early-season momentum by seeing off Beatriz Haddad Maia 7-5 6-4.
Ninth seed Jessie Pegula – a quarter-finalist in the season's opening grand slam – beat doubles world number one Katerina Siniakova in straight sets, while former Australian Open champion Sofia Kenin lost in three sets to fellow American Ann Li.