Unseeded Katerina Siniakova ended Mirra Andreeva's Indian Wells title defense on Monday, rallying for a 4-6, 7-6 (7/5) 6-3 victory over the eighth-ranked Russian.
The 18-year-old Andreeva had opened her repeat bid with an imperious 6-0, 6-0 demolition of Solana Sierra.
But she was in trouble early and often against 44th-ranked Siniakova in a rollercoaster contest that ended with a shot from Siniakova that hit the net cord and dribbled over in one last frustrating moment.
The 18-year-old Russian slung her racquet as she approached the net before departing the court with a defiant gesture at the censorious crowd.
Siniakova, a former doubles No 1, admitted it was an awkward way to seal the win in a match that featured seven service breaks for each player and 43 break chances between them.
"Of course I'm happy it went on the other side," she said. "I was, like, should I cheer? It's a really tricky finish. But definitely I will not say I'm not happy."
Siniakova said the swirling winds on Stadium Court were troubling both players.
Andreeva's emotions had already boiled over when she sailed a swinging volley long to surrender the second set, during which each player had remarks for the chair umpire about her opponent.
She regrouped to break Siniakova for a 3-2 lead in the third, but Siniakova won the next four games, saving two break points in the final game before gaining the win on her first match point.
Siniakova will face Elina Svitolina for a place in the quarterfinals after the ninth-seeded Ukrainian beat American Ashlyn Krueger 6-4, 6-2.
There was no drama for world No 2 Iga Swiatek, who defeated Greece's Maria Sakkari 6-3, 6-2.
Poland's Swiatek beat Sakkari in the final to claim both of her Indian Wells titles, in 2022, and 2024, but Sakkari had won their most recent encounter in the quarterfinals of the Qatar Open last month.
Swiatek said she had learned from "obvious mistakes" she made in Doha.
"I knew what to focus on today and I did that from the beginning till the end," she said. "So I'm really happy with my focus and the way I was prepared.
Swiatek next faces Doha champion Karolina Muchova, who blew past Antonia Ruzic 6-0, 6-3.
In other early matches, fifth-seeded American Jessica Pegula shook off a slow start to beat Latvia's Jelena Ostapenko 4-6, 6-3, 6-2.
Pegula, coming off her fourth career WTA 1000 title at Dubai last month, fired 11 aces with just one double fault as she rallied for the win.
"I think today I had to kind of snap myself back and kind of lock in to not let that get away from me," said Pegula, who said she was in danger of letting negativity and frustration get the better of her.
Pegula next plays Switzerland's Belinda Bencic, a 6-2, 6-3 victor over Elise Mertens.
Australian Open champion Elena Rybakina, who lifted the Indian wells Trophy in 2023, faces Ukraine's Marta Kostyuk in the final match of the night.