LONDON, UK - At the age of 28, Petra Martic feels that she is coming into her own. This season has seen the Croat tick off a number of long-desired milestones: a maiden title in Istanbul in April was the precursor to a first Grand Slam quarterfinal run at Roland Garros last month.

Since March, Martic has compiled an 18-4 record in total, carrying her form over to grass in style to reach the Birmingham semifinals last week, and hit a career high ranking of WTA World No.24. Her upsurge, she says, is validation of an unusual coaching choice she made two years ago, just as she was returning to the tour after several years beleaguered by injury: to hire fellow player Sandra Zaniewska full-time. 

It's a rare professional partnership in a number of ways: not only is the Pole one of the still all-too-rare travelling female coaches on tour, but at 27 years old she is in fact younger than her charge. In Paris, Martic recalled the scepticism she had faced after hiring Zaniewska: "When I started working with her, everybody was doubting my decision. People underestimated it, people even laughed at it, said I did a crazy thing, what am I doing? I'm not serious about my career because she's so young and inexperienced."

Sandra Zaniewska celebrates charge Petra Martic's run to her maiden Grand Slam quarterfinal at Roland Garros 2019 (Jimmie48/WTA)

In 2017, the duo's careers overlapped going in opposite directions. Martic was in the process of essaying a remarkable comeback, shooting from WTA World No.662 to No.89 in just three months thanks to fourth-round showings at Roland Garros and Wimbledon.

Zaniewska, by contrast, was finally calling time on a career that had also been beset by injury: having reached a career high of WTA World No.142 in 2012, the year she qualified for her only Grand Slam main draw at Wimbledon, she would play her final match in Ilkley in June 2017.

Read more: Petra Martic relishing 'second chance' in Paris

Initially, Zaniewska - who had never coached before, and had no intentions to start beyond a jobbing part-time way to earn extra money as she returned to studying - was just a background voice offering advice to Martic. But after six months, Martic, who blends power and finesse to make for one of the most well-rounded and dangerous skillsets on tour, realised how valuable Zaniewska's input had been.

"I knew she would understand my game better than maybe other coaches who had more experience," says Martic. "When she played she had a similar style to mine - "

Zaniewska interjects: "But much worse!"

Smiling, Martic continues: "The game I have is not always easy, because I have many options on the court - so for me to choose the right option was always challenging. It took me a long time for me to figure out what I want on the court, and she really narrowed my options so it's easier for me to win."

For the last two years I would say that I've really figured out what I want because of her. She helped me find my identity, and only now I see my game as an advantage.

- Petra Martic on coach Sandra Zaniewska

Martic's ability to hit any shot in the book is, to most fans and opponents, one of her signature strengths. But the player herself hasn't always seen it that way.

"[Developing] it was very hard," she remembers. "Some coaches wanted me to decide on my own what to do, some wanted to do points the way they wanted, and at the same time I didn't know what I wanted to do or how - what my identity on the court was. 

"For many years I was actually really lost with this game, and as much as people saw it as my advantage, I saw it as a ridiculous challenge that I was never gonna overcome. But for the last two years I would say that I've really figured out what I want because of her. She helped me find my identity, and only now I see my game as an advantage."

Petra Martic and Sandra Zaniewska train together at the Nature Valley Classic in Birmingham (Jimmie48/WTA)

Zaniewska has also been able to turn her career-ending injuries into a source of advice as the two have sought to maintain Martic's health. "I was injured many times because of the fact that I wasn't listening to my body," she recalls. "I always tell her, listen to it, it's really good - it's better that you listen to it than not at all, because I know how that can go - in a bad way."

The partnership between two players so close in age also makes for a refreshing on-court dynamic. Often, coaching relationships contain an underlying power imbalance: either the coach as an authoritarian father figure ordering his charge around, or the player as a strict employer bossing an underling. Martic, who says she has experienced unhealthy coaching dynamics in the past, sees her collaboration with Zaniewska as one between equals. 

"I think it's the best way to do it," she says. "If one is dominating it's not going to work because the other will be unhappy. I had it where I was just listening to the coach and I didn't have any input. I would lose a match and I'd just be sitting waiting for the answers. It's not healthy if in practice you're not giving your input - it means you're also not following what's happening, you don't pay attention, and then when you actually need to do it yourself, you can't.

"We're both human beings and we try to treat each other as such. Coach and player is a secondary thing. This is for us the most important thing, to have respect towards each other, to be honest and to do good work."

We're both human beings and we try to treat each other as such. Coach and player is a secondary thing.

- Petra Martic on her partnership of equals with Sandra Zaniewska

All of this is despite the fact that Martic and Zaniewska describe themselves as "entirely opposite" as people.

"If I like something, then I already know she's not gonna like it," grins Martic. "Opinions about things, things we like to do - it's every single time and now we just laugh about it."

One example is the subject of on-court coaching, which Martic thinks should be uniform - either in its presence or absence - across both WTA tournaments and Grand Slams, but which Zaniewska is firmly against. (In practice, Martic says that while she wants to be mindful that she needs to do without it at majors, "it's harder in the moment" to resist when she realises a few words from Zaniewska can set her right mid-match.)

"We're pretty different, but we manage to put it together," sums up Zaniewska of the unconventional path the pair are forging.

"I think it's nice," adds Martic. "It's not a fight - yet! - and we broaden our horizons."

With even more success, the partnership could broaden the horizons of many more players about what a coach-player relationship can be.