World number three Garbine Muguruza crashed out of the US Open as she was stunned in round two by Latvia's Anastasija Sevastova.
Muguruza won three games in a row to threaten a comeback in the second set but Sevastova finally converted her fourth match point to win 7-5 6-4 in Arthur Ashe Stadium.
Sevastova temporarily gave up tennis in 2013 due to illness and injury but she has now reached her first grand slam third round, where she will meet Kateryna Bondarenko.
"I was shaking a little bit at the end," Sevastova said. "It's amazing. On Ashe, in a night match, what's going to be bigger?
"I had to solve my head, my thoughts, what could be, what could not - everything. But I am so happy it went well."
The world number 48's only other match against a top-10 player this year came against Simona Halep in July, when she lost 6-0 6-0.
The Latvian stuttered when leading Muguruza 5-1 in the second set, before breaking the Spaniard to finally get over the line.
"I didn't feel calm inside," Sevastova said. "Then I stopped thinking. I was thinking too much at 5-2, at 5-3, then I thought I had one more chance at 5-4.
"I thought I broke her enough times so I tried one more time."
Asked what she would do to celebrate, Sevastova said she would go out with her mum, who was watching her daughter for the first time at a grand slam.
"She will drink," Sevastova said. "And I will watch."
Muguruza has now failed to make round three at Flushing Meadows in four attempts and she has also bowed out in consecutive grand slam second rounds after going out at the same stage at Wimbledon.
It represents another disappointing result for the 22-year-old, who has struggled to back up her impressive run to the French Open title in June.
"I think it was a very tough match. I think I didn't play well today. I think she also played well. Everything she was doing was kind of working," Muguruza said.
"I don't think about the French Open any second. It's past. Everybody's looking forward to the next one, the next tournament.
"Of course you have in your mind it's a great achievement. No matter what, it's a grand slam trophy.
"But I'm not thinking at all when I come here that I won the French Open. This is new tournament."
Former world number one Caroline Wozniacki also recorded an upset as she came from 4-0 down to beat ninth seed Svetlana Kuznetsova.
Wozniacki made a nightmare start and was two points from trailing 5-0 in the first set, but she came storming back to secure a surprise 6-4 6-4 victory.
The Dane climbed top of the world rankings in 2010 but has struggled for fitness and form since and now finds herself placed 74th in the world.
Twice a US Open runner-up, Wozniacki has been sidelined by ankle and arm injuries this season but the 26-year-old insists she never considered retirement.
"I wasn't thinking about quitting. I know that I have a lot of opportunities, I have a lot of other interests, and my life is going to be good regardless," Wozniacki said.
"Honestly I just have fun now. I enjoy playing. I love playing tennis. That's why I play. At the end of the day, I know my worth. I know how good I can play."
Britain's Johanna Konta admits she pushed herself to the limit after surviving an on-court collapse.
Konta beat Bulgarian Tsvetana Pironkova 6-2 5-7 6-2 to reach the third round in New York, but only after enduring a sudden bout of illness on Court 13 that left her lying in a heap on the baseline.
The British number one was a set point down in the second set when she dropped to the floor, red in the face and struggling to breathe, with almost 15 minutes passing before play resumed.
The match began just before 2pm local time at Flushing Meadows and it is likely the heat, reaching 30 degrees Celsius, contributed to Konta's discomfort.
"I think all athletes kind of play a very fine line with that. We push our bodies to the limits that they can go," Konta said.
"I definitely hit one of my limits. That was the way my body reacted.
"I was feeling a little bit the conditions and also my own energy levels. I was just managing the best that I could with what I had today.
"Basically my heart rate spiked and I couldn't really control my breathing. I started hyperventilating. I started shaking. So yeah, basically that's why I went down on the ground, because I was quite violently shaking."
Elsewhere, Angelique Kerber, Petra Kvitova, Roberta Vinci and Madison Keys all made it through in straight sets.
