Unranked Bulgarian Tsvetana Pironkova extended her US Open giant-killing run on Saturday, scoring a second consecutive upset with a straight-sets defeat of 18th seed Donna Vekic.
Pironkova, who is playing her first tournament in three years, polished off Croatia's Vekic 6-4, 6-1 in just over an hour.
The 32-year-old, who had eliminated 10th seed Garbine Muguruza in the second round, will face either seventh seed Madison Keys or France's Alize Cornet in the last 16.
Pironkova's run to the fourth round equals her best performance at the US Open. She reached the last 16 in 2012, where she lost to Ana Ivanovic.
Pironkova only returned to tennis this season after taking a three-year break from the WTA Tour, during which she gave birth to her son in 2018.
Pironkova said Saturday she had contemplated giving up tennis altogether during her extended layoff.
"There was a time after I became a mom where I didn't really picture coming back to the tour at all," she said. "I was happy with the moment, just wanted to live through everything, through this new exciting times in my life.
"But then I started thinking about tennis more and more. My first tournament is actually main draw in US Open. That is a huge opportunity.
"At one point I was like, I do not want to miss that opportunity, I'm going to start training, I'm going to do everything I can, and it's going to be different this time because, first and before all, I kind of enjoy it because tennis is before all a game."
Because of her long absence from the sport, and the disruption caused this season by the coronavirus pandemic, Pironkova does not have a world ranking.
Her last tournament appearance before this week's event at Flushing Meadows came at Wimbledon in 2017, when she exited in the second round.
Pironkova's best performance in a Grand Slam came in 2010, when she reached the semi-finals at Wimbledon.