Tsvetana Pironkova reached the quarter-finals of the US Open on Monday, defeating France's Alize Cornet in three sets.
The unranked Bulgarian, who only returned to tennis this year following a three-year absence from the circuit, secured her last eight berth with a 6-4, 6-7 (5/7), 6-3 victory at the Louis Armstrong Stadium.
Pironkova will now face third seed Serena Williams in the quarter-finals on Wednesday.
"It's absolutely unreal, I really can't believe it," Pironkova said.
"Especially that I pulled through at the end of the match -- I was exhausted.
"I didn't think I had it in me but I just kept fighting and fighting and it paid off."
It is only the fourth time in Pironkova's career that she has reached the quarter-finals of a Grand Slam singles event, and marks her first quarter-final appearance at the US Open.
Pironkova's last tournament appearance before this year's US Open had been the 2017 Wimbledon championships when she exited in the second round.
She gave birth to her son during her three-year break from tennis but elected to return in 2020, only to see her comeback plans disrupted by the coronavirus shutdown.
In New York she has claimed a couple of seeded scalps, defeating 10th seed Garbine Muguruza in the second round before knocking out 18th seed Donna Vekic in the third round.