"It still hasn't settled in," she said in her post-match press conference. "I mean, I'm tired mentally and it's late. Normally I go to sleep at this time.
"It feels great, but it's still not like I won the tournament. It's only second round."
Sevastova was a rising star back in 2011, reaching the fourth round of the Australian Open at 20 years old. But injuries soon sapped the desire away from the young Latvian, who hung up her racquets two years later, and was off the tour for two years more.
"I was playing a little bit. I was doing some sparring. It was better, physically. I was still playing okay. That's why I think I decided to try again to come back.
"But you never know. A lot of people are trying to come back, and sometimes it doesn't work out. That's why I wanted to start from the beginning, from the lowest level."
Playing on the game's biggest stage, Sevastova thrilled the Arthur Ashe crowd as she took the first set and raced out to a 5-1 lead in the second, holding two match points in the next game.
"I stopped thinking. I was thinking too much at 5-2, 5-3. Then I said, 'Okay, I have one more chance at 5-4. She's serving. I broke her before enough times, so I will try this one more time.'
"And still, even then it's only 5-5. It's still an open match like the first set."
That positivity took the Latvian over the finish line, breaking serve to love to clinch the biggest upset thus far.
"I think it was a very tough match," Muguruza said after the match. "I think I didn't play well today. I think she also played well. Everything she was doing was working. Maybe at the last part of the match I could feel like I could have a chance to come back, but, in fact, it was like 50/50.
"So at the end she played well, and that's it."
Still, the Spaniard seemed to take the defeat in stride, joking about her less-than-stellar record at the US Open.
"I try to give my all. I try to fight on court. She was just better. I don't know if it's bad luck.
"But, well, I did my best result again! I'm just saying that."