MONTRÉAL, Canada - It seemed like the best of times for Kiki Bertens last fall, finishing 2017 on a high when she made the doubles final at the BNP Paribas WTA Finals Singapore presented by SC Global.

But even as she completed a breakthrough season in singles, one that saw her crack the Top 20 for the first time in her career, the 26-year-old knew she needed to make a change.

"Last year I was really struggling," she said after a milestone victory over Karolina Pliskova - her first over a Top 10 player on hardcourts - at the Coupe Rogers. "I had a really good year last year, but at the end I was just exhausted."

A bright start on her beloved clay - where she strung together solid runs at the Mutua Madrid Open, the Internazionali BNL d'Italia, and a title at the Ladies Championship Gstaad - gave way to disappointments on hardcourts that left her wondering how to attack the looming 2018 season.

"I had to make a decision at that time, and it was just really tough on me. But I said, ‘Okay, I want to continue, but in my way.’ So I made some rules for myself, how I want to play, and how I want to be on tour. I wanted to become a little bit more relaxed, and that’s helping a lot."

An off-season holiday helped ready her for the intense bloc that was to come with longtime coach Raemon Sluiter, who allowed Bertens, now 26, to take a more collaborative role in the training process.

"We had a really good meeting, because we had begun to doubt our partnership a little, like, 'Should we continue or not?' But once we sat down together and made some rules, he was really happy with the way I came up with some ideas, which I hadn’t done before, so that was quite new for me. We started again fresh, and we’re still working.

"I’m much more fit now. I think we trained really hard, I lost some weight, and the movement is really important for me. I think in general, my level just got higher. I think before I could play really well, but only when the level was really good.

"Now, even on the days that I don’t play great, I’m still managing to get the good level, and I think that’s big progress I’ve made."

Unsurprisingly, the work began to show once the 2016 French Open semifinals slid onto clay, winning the biggest title of her career Volvo Car Open. But the Dutch powerhouse hit new heights in Madrid, where she beat Caroline Wozniacki, Maria Sharapova, and Caroline Garcia en route to her first Premier Mandatory final.

"My level just got higher. I think before I could play really well, but only when the level was really good. Now, even on the days that I don’t play great, I’m still managing to get the good level, and I think that’s big progress I’ve made."

- Kiki Bertens

The physical improvements have not only made her stronger, but also mentally tougher, helping to Bertens tackle the doubt that sometimes haunted her in big matches.

"[Being fitter] means that you don’t have to play really aggressive all the time, that you can get a lot of balls back in play, and go from there again. I think that helps me a lot."

That more moderate mentality took her into a career-best grass court season, where she stunned five-time Wimbledon champion Venus Williams to make the quarterfinals, a result that made her re-evaluate how she prepares for the US Open Series.

"I always liked to play a long clay-court season, so I’d play the tournaments after Wimbledon on clay, and then took a rest, and then start in Cincinnati. So that was more of my schedule. Now I changed it, I skipped the clay-court season after Wimbledon because I did well there."

Back on hardcourts for the first time since narrowly losing to Venus at the Miami Open, Bertens has dropped just seven games in two matches to book a Madrid final rematch with Petra Kvitova.

"I’m just happy with the way I’ve progressed my game on the hardcourts, I think since this year, it’s going better, my movement is better. It’s a new challenge, I would say."

A strong run in Montréal would likely supercede her current career-high ranking of No.15, but in a season of change, that one might be the easiest to make.

"I'm at a point where it's like, 'Okay, it’s just tennis.’ It’s a part of my life, of course, and a really big part, but that’s not my life. So, I can still lose a match and feel good afterwards.

"I think that’s more like the kind of mental change you have to make, and of course, on the court, you have to go for every shot, go till the last ball, but at the end of the day, it’s just a match."