NEW YORK, NY, USA - Tennis often came easy to Donna Vekic, who smashed the WTA ceiling as a teenager to crack the Top 100 and win a title within her first 12 months on tour.

GALLERY: Great Escapes - Vekic wins from match point down for the third time in 2019

Sustained success, however, proved elusive until the Croat made the changes necessary to reach her first Grand Slam quarterfinal at the US Open.

"With me, it always has to be a little bit complicated," Vekic joked after saving a match point to survive 2018 Wimbledon semifinalist Julia Goerges on Monday.

The 23-year-old has rallied from match point down a total of three times this season, a stat that underscores her vastly improved fitness - "I felt like I could play for another five hours" - and, according to coach Torben Beltz, just how match tough his student has become.

"It’s tough to prepare for this, but if you play many close matches, you stop looking so much at the scoreboard," he explained in his WTA Coaches press conference. "You start staying more in the moment, grind hard, and play every point tough.

"Today, Julia was serving for the match, had match points, and Donna told herself she had to play every point tough to have a chance to change the game."

Change has been the optimal word for Beltz, who coached former World No.1 Angelique Kerber to two major titles in 2016. Splitting with Kerber at the end of 2017, Beltz was resolved to find a promising young player, one who could match the peaks of his previous partnership.

"When I went to Geneva for our first talk, it was about what we can improve. I said it was going to be about footwork, and also about getting a bigger game.

"Angie and Donna are really different players. For Angie, it was more about counterpunching, relying on footwork. For Donna, I want her to be more aggressive, to serve bigger, and to play a bigger first shot. I had to change my approach, to talk about working on making the first shot more aggressive and maybe coming into the net more."

Armed with a fearsome serve and forehand, Vekic spent years stuck in the middle of the Top 100 following her initial breakthrough - bouncing between No.60 and No.110, bottoming out at No.177 in 2015. She was on the way back up before pairing with Beltz, and has only gotten stronger since, reaching the second week of Wimbledon and Roland Garros to close in on the Top 20 for the first time this spring.

"I think the overall game is better. There’s more power in the groundstrokes. Her footwork is better, and it still can improve, which is good for me, as a coach, to see that there’s still some room for improvement. She moves better, has better defense than before, and she can find a way to go from defense to offense.

"In San Jose, we played against Misaki Doi, and a coach told me that Donna is playing more and more like Angie. I was like, ‘What? Really?’ But when it comes to footwork and defense, those are both much better now."

For all of that forward momentum, Vekic set modest goals at the start of 2019, hoping to qualify for the Hengqin Life WTA Elite Trophy Zhuhai. She scored two wins over Victoria Azarenka at the Mubadala Silicon Valley Classic and Western & Southern Open, where she pushed five-time Wimbledon champion Venus Williams to three sets.

"After Wimbledon, I was pretty nervous because the first half of the season was very good for me, and I was thinking, ‘Don’t mess this up now!’" she said after reaching the second week. "I was really nervous in San Jose and Toronto, as well, but in Cincinnati, I found my rhythm again and I’m playing good tennis here."

Making her fifth main draw appearance in Flushing Meadows, she posted impressive wins over tricky opposition like Richel Hogenkamp, "US Open specialist" Kaia Kanepi, and Yulia Putintseva before facing Goerges - a player whom she'd never beaten - to book a French Open rematch with good friend Belinda Bencic in the quarterfinals.

"I will probably try to change the rhythm and play a few high balls, few slices, few dropshots to try to keep her guessing, but my main game will definitely be to be aggressive and to try to open up the court."

Vekic is just 10 months older than Bencic, the reigning Dubai Duty Free Tennis Championshiips champion who reached the US Open quarterfinals back in 2014, and admitted she likely wasn't ready for the sort of success both initially enjoyed.

"I was Top 100 when I was 16, and putting so much pressure on myself. I thought I would be making finals every week, and that was just not the case. There was still so many improvements to make and so many things to work on.

"I definitely wasn’t aware of that when I was 16. There are a lot of amazing young girls coming up, and I hope they have a good team around them who’ll know how to guide them through tough times."

With Beltz by her side, the No.23 seed is enjoying a proper breakthrough for the first time in her still-young career, and the time it took for big results to come might ultimately help her shine beneath the spotlight.

"If this happened to me when I was 18, I would be, like, 'Oh, my God, this is unreal.' But right now I'm really happy, but I'm not done. I don't want to be done. I feel like there is still a lot to be done here, and I really want to keep going and try to win a few more matches."

"She’s never had a big score, like winning Indian Wells or Miami," Beltz added, "but third rounds, fourth round, quarterfinals, semifinals, and now we need, for the next step, to make a big score somewhere.

"Maybe at the US Open!"