Hearing there's a little bit of a delay at Flushing Meadows. Serena Williams is on site, we've seen her arrive sipping coffee, complete with pooch tucked in her bag, but she's not yet ready to join Pironkova on court for the warm-up..

Here's what Tsvetana Pironkova, who was ranked 131st in the world when she last played in 2017, has made of reaching the quarter-finals at Flushing Meadows: "It's absolutely unreal.

"I really can't believe it. I was so exhausted by the end of the (last) match I really didn't believe I had it in me. I kept fighting and it obviously paid off."

Though, she is missing her son...

"I haven't seen him in two weeks," she said. "It's very tough and it gets tougher every day.

"I know he's watching me and I know he's proud of me. It's worth it."

Image source, Getty Images

So, what do you know about Tsvetana Pironkova?

Wimbledon 2010, anyone? The Bulgarian enjoyed her best run at a Grand Slam a decade ago at the All England Club, beating Venus William en route to the semi-finals, where she lost to Vera Zvonareva.

But her current form is perhaps more remarkable - at 32, Pironkova has returned from a three-year hiatus from the sport to reach the US Open quarter-finals for the first time in her career.

The former world number 31, who last played at Wimbledon in 2017, considered not heading back to the tour following the birth of her son in 2018.

She'll be glad she did make a comeback in March this year, after new ranking protection rules allowed players going on maternity leave to enter 12 events on their previous ranking.

Jennifer Brady and Naomi Osaka have already booked their places in the US Open semi-finals, now it's up to Serena Williams and Tsvetana Pironkova to see who will grab the first spot on the other side of the draw.

Grab some popcorn, it should be a cracker...