"The feeling is unreal, and I still can't believe this match ended with a win for me," she said after the match.
"I was trying to be focused because she's a great player with great experience coming back in many amazing matches. In the end, it was just point by point."
Working with coaching consultant Justine Henin, a former World No.1 and recent International Tennis Hall of Fame Inductee, Svitolina has shown steady signs of improvement in the last two seasons, reaching her first Grand Slam quarterfinal and peaking at No.14 in the world following a title run at the BMW Malaysian Open in Kuala Lumpur.
But Svitolina had taken just one set from Serena in their previous four meetings, and was by all accounts the underdog against the four-time Olympic gold medalist.
Navigating through tense opening exchanges, Svitolina recovered from losing her early break advantage to reclaim the initiative and serve out the opening set in 34 minutes.
Serena w/d from Montreal citing shoulder inflammation. Could barely spin a serve at the end of the 2nd set. Can she bounce back @usopen?
? WTA Insider (@WTA_insider) August 9, 2016
The second set saw a struggling Serena, one who appeared to aggravate a right shoulder injury that forced her out of the Rogers Cup last week, roar back from a break down to level the contest at three games apiece, but it wasn't enough on Tuesday night.
"In that moment, I was just trying to stay in the moment and focus on every point. She was struggling a bit, and I could see that if I pushed a little more, I could be on top. This was the key."
Serena had given kudos to her opponent following her second round win, something which made the win all the sweeter for Svitolina, who grew up idolizing the World No.1.
"It's amazing, and one of my dreams to play against her. I don't think I ever dreamed of beating her. I think I'll enjoy this moment so much, especially at the Olympics; I've always wanted to play here."
Up next for the Ukrainian is No.11 seed Petra Kvitova, who is hoping to better her quarterfinal finish from the London Olympics in 2012. Kvitova won a titanic three-setter over Ekaterina Makarova earlier in the day.
? ITF Olympic Tennis (@OlympicsTennis) August 9, 2016