NEW YORK, NY, USA -- Tsvetana Pironkova’s incredible comeback run at the 2020 US Open continued unabated on Saturday, as the Bulgarian zipped past No.18 seed Donna Vekic of Croatia, 6-4, 6-1, to reach the fourth round of the event for the second time in her career.

"It feels great," Pironkova told the media, after her third straight-set win of the event. "It feels amazing. Of course, unexpected. But just super happy."

What makes the accomplishment so stunning is that this is Pironkova’s first professional event of any stripe since her most recent match at 2017 Wimbledon. The 2010 Wimbledon semifinalist was off the tour for more than three years while raising her son Alexander, who was born in April of 2018.

"It feels the same to me, it feels normal to me," Pironkova stated about returning to the tour. "I've been doing that for more than 15 years. I just had three years' break. That's kind of how I grew up, what I have been doing for a long, long time. You cannot just forget it like that. It just doesn't happen that way, I guess."

Read more: Pironkova returns to tour: 'I'm glad I took the challenge'

After an absence of over three years, Pironkova came into the event unranked, using her special ranking after maternity leave to enter the main draw. Her decision has paid off, as she has won two straight matches against Top 25 opposition, following up her second-round win over No.10 seed Garbine Muguruza with the breezy 63-minute victory over Vekic.

"Having this opportunity of the special ranking gives you more motivation, as well, because you're not starting from scratch, as we can see," said Pironkova. "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. It's interesting, exciting, and super cool game that you can have a lot of fun playing."

"I'm good friends with Kateryna Bondarenko who is now a mother of two, and she's also back on the tour after she gave birth to her second child," Pironkova added. "She was sharing her experience, how that works. It's helpful to have fellow mothers on tour."

The Bulgarian came into the event undefeated against Vekic, having won their three previous meetings, which all took place prior to 2017. Pironkova continued her dominance in their rivalry, converting all four of the break points she held, and winning nearly half of the points when Vekic was serving.

Vekic, a US Open quarterfinalist last season, was undone after losing a close first set, firing 16 unforced errors, including six double faults, in the second set. Pironkova, by contrast, had just two unforced errors during that set.

"It was a tough match, I would say, even though the score does not suggest it that much," said Pironkova. "The first set was really, really close, especially around 4-4. It was a matter of just a couple of points really. I'm really glad that I got to hold on, to take that set. Then in the second set, there were just a few moments that were important, as well. I was just keeping persistent, and that kind of paid off."

Two love holds kicked off the match, but things would get much more complicated immediately after that start, as Vekic had to serve her way out of a protracted game to hold for 2-1, while Pironkova needed to save a break point to match the Croat for 2-2.

Errors began to creep into the Vekic game as she fell behind a break for 4-3, but the seeded player regained control in the following game as she reclaimed level footing at 4-4. In the next game, though, the double faults by Vekic started to rear their head, as two in a row showed up to give Pironkova break point. A long forehand by Vekic ceded the 5-4 lead back to the Bulgarian.

Serving for the set, Pironkova smoothly fired two service winners to kick off the game, en route to double set point. On her first chance, Pironkova was rewarded with a long miscue by Vekic, and the unranked mother was a set away from the second week of a Grand Slam event for the first time since her quarterfinal run at 2016 Roland Garros.

That set, as it turned out, came easily for Pironkova. After a hold by Vekic to start the set, Pironkova reeled off six straight games, using pinpoint groundstrokes to power through points, while Vekic continued to struggle with avoiding errors.

Two double faults at 1-1 and three more at 3-1 put Vekic in a dire situation, as she quickly fell behind 4-1. A backhand winner to hold for 5-1 led Pironkova to match game, and on her first match point, a deep forehand helped set up a winner down the line to close out the match and keep her magical fortnight alive.

Pironkova now finds herself into the US Open round of 16 for the first time since 2012, having not dropped a set in her first three matches. Her next opponent will be the winner of Saturday night’s Arthur Ashe Stadium showdown between No.7 seed Madison Keys of the United States and Alize Cornet of France.

"Both opponents, Alize and Madison, are amazing players," said Pironkova. "I have played against them both. Alize is a big fighter. She's always fighting until the last point. Madison has big shots, big serve, big forehand. It will be a challenge no matter which one of them it will be."

Two other Top 20 seeds were more fortunate than Vekic on Saturday, as No.16 seed Elise Mertens and No.20 seed Karolina Muchova booked fourth-round spots, although they took differing paths to get there.

Mertens of Belgium quelled rising American teenager Caty McNally, 7-5, 6-1, to reach the fourth round of the US Open for the third straight season.

Mertens, a quarterfinalist at the US Open last year, has now won 22 matches in 2020, tied for the most on tour with Elena Rybakina. The Belgian, though, was made to work by the aggressive-minded McNally before claiming the win in an hour and 24 minutes.

McNally led 3-0 in the opening frame before Mertens righted the ship, methodically pulling her way back to eke out the one-set lead. The World No.18, a finalist in Prague and a semifinalist at the Western & Southern Open since the tour’s resumption, continued her solid play, winning 83 percent of her first-service points and slamming 22 winners to get another win.

Things were much less straightforward for Muchova of the Czech Republic, who was on the brink of defeat before squeaking out a 6-3, 2-6, 7-6(7) nail-biter over Sorana Cirstea of Romania, saving three match points in the final-set tiebreak.

The Czech, who eliminated two-time US Open champion Venus Williams in the opening round this week, needed two hours and 17 minutes to quash the challenge from former Top 25 player Cirstea and reach the fourth round at the event for the first time, following third-round losses in the last two years.

World No.26 Muchova, a quarterfinalist at Wimbledon last year, eased through the first set before 77th-ranked Cirstea stormed back in the second set. The Romanian reduced her errors from 13 in the opener to just three in the second set.

Cirstea started the third set with momentum, twice going up a break as she served for the match at 5-4. However, the Czech struck back and the pair advanced to the decisive tiebreak. The Romanian took a commanding 4-0 lead in the breaker, and though Muchova pulled back on serve at 4-3, Cirstea claimed two points in a row to lead 6-3 and reach triple match point.

Bit by bit, the aggressive Muchova erased each of those chances, reeling off four points in a row to clinch her own match point at 7-6. The Czech missed her first chance, but earned another one at 8-7, which she converted to steal the comeback win.

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