SINGAPORE - Raemon Sluiter has vivid memories of the last time he was in Singapore. It was here last year at the BNP Paribas WTA Finals Singapore presented by SC Global that he issued Kiki Bertens an ultimatum and some tough talk. 

Then ranked No.31 in singles, Bertens qualified for the WTA Finals in doubles with Johanna Larsson, and the two made a strong run to the final. What should have been a happy end to the season was anything but. By all accounts, Bertens had been unhappy on tour and struggled to put in the consistent effort to get the most from her singles career, and Sluiter was at the end of his rope. So he told the 26-year-old, maybe it's time to hang up your racquets. 

"After the Singapore Finals here, we were sitting in Kiki's room with her boyfriend, and then I said, well, if we continue like this, this is it for me, and I strongly advise you to go and do something else," Sluiter told reporters ahead of the BNP Paribas WTA Finals Singapore presented by SC Global. 

"Because I have seen you over the last four or five months. The only times I saw you smile were at home and never when we are on tour. You're making good money. But it's not bringing you anything apart from that. And no money is worth feeling miserable all the time. And that was really the case with her. 

"So I said, In order to do this, for me, things need to be different, but that's not the most important. The most important thing is for you, because this is your career. And would you be able to quit tennis now, sitting at home, and not thinking in three or four years, Maybe I should have done things a little bit differently? Can you live with that?"

"Would you be able to quit tennis now, sitting at home, and not thinking in three or four years, Maybe I should have done things a little bit differently? Can you live with that?"

The question was left unanswered when the two went their separate ways for the off-season. Bertens thought long and hard about whether to even continue her career, but Sluiter's question resonated.

"After two weeks of holidays, the answer was, No, I cannot live with that, and I'm going to try and do things differently.

"She wrote down a few points, not necessarily about the points, but she took matters more in her own hands. Right away when I saw the paper, and actually from the moment she said I wrote down a few points, I said, Okay, I'm in. Let's do this.

"Because I saw she wanted to do things differently. And that's what she's done."

Kiki Bertens receives the 2018 Most Improved Player award from Jennifer Capriati and Steve Simon. (Getty Images)

The result has been an astounding transformation, as the Bertens, who had always accepted that she was little more than a clay-court specialist, evolved into an all-court threat. Her season, which included two Premier titles at the Western & Southern Open and Volvo Car Open, the biggest final of her career at the Mutua Madrid Open, and a run to the quarterfinals at Wimbledon, solidified the Dutchwoman as a major contender, even more so than when she made the French Open semifinals in 2016.

No one has tallied more Top 10 wins than Bertens this season, and no player in the Singapore field won more matches after Wimbledon than her. And for the first time in her career, Bertens tallied over 40 wins in a season en route to making her WTA Finals singles debut. 

But the philosophical Sluiter, himself a former ATP player, says he's less wowed by Bertens' results than with her new work ethic. 

"It's been a great season, especially work-wise," Sluiter said. "That's been the problem. We have been working for three years, and the other two seasons were good in their own way, but work-wise, they were a little bit up and down.

"Kiki had a little bit of a tendency after a tough loss to go away for a few days, practicing less, not focusing on food, on what she needed to do, and therefore those seasons were pretty up and down. Your season is always going to be up and down, but to get it less up and down, you have to focus on the stuff you can control.

"That's what she has done really, really well this year. That's what I'm most proud of, because I know it's not natural for her."

Kiki Bertens is the third Dutchwoman to ever qualify for the WTA Finals, along with Betty Stove and Brenda Schultz-McCarthy. (Getty Images)

Also not natural for the affable Dutchwoman is the spotlight. As she readied herself to be announced to the stage at the glamorous draw ceremony on Friday night, Bertens winced and muttered "Wow" when the bright spotlight shone down on her. It was a small moment but one that that offered a window into just how much that side of being a professional athlete can leave Bertens unsettled. Attention has never been the reason Bertens picked up a racquet and it's certainly not the reason she continues to play.

"Before, I think it was also a fight for her inside, because as you can see, she's doing this stuff, this media stuff, a lot better now, but it's definitely not her thing," Sluiter said. "You know, she will be nervous for the match but not as much as that 20 meters on that red carpet she needed to do just before the draw. So we had the worst already this week, which is a very good thing.

"So I think that has always been a battle for her inside, because she knows if you get better, this stuff is going to be more with that. I just try to keep it a little bit more simple. We'll see it when it comes, you know. 

"But for me, always the most important thing was, like I said, when you are at home in a few years or three or five or ten, can you say to yourself, I have done everything? Because it's the thing that I regret the most for my own career. 

Raemon Sluiter reached a career-high No.46 in 2003. (Getty Images)

"I think I played okay tennis. I had a great time. I had too good of a time probably every now and then on the tour, which is also worth something, but I wish I could have done things a little bit different. And that's the one thing I don't want for her."

Bertens credits the hard work she put in the off-season on her physical conditioning and improved discipline for her turnaround. She's also taken up yoga, which helps her calm her mind and handle the on-tour stress that made her life miserable in years past. Bertens and Sluiter both say she plays her best when she simply focuses on playing the ball. It's when she becomes overwhelmed by the pressure to perform and achieve that things begin to fall apart. 

"Now she's more in the moment," Sluiter said. "That's been a big part of this season, being in the moment when you are playing well but especially try to be in the moment when you are losing and when things are not going your way.

"I always try to compare it with a house or hotel that's burning. You need to know the exit. You need to know where the exit is. If you don't know it, you're going to panic. And with panic, it's not going to be in your system. You do something, but it's not going to be in the system."

Kiki Bertens has posted a tour-best 10-5 record against Top 10 players in 2018. (Jimmie48)

Staying in the moment was a major obstacle the last six weeks as she found herself chasing Singapore qualification and a Top 10 debut. She bolstered her chances by winning her third title of the season in Seoul, but the neck and neck battle between herself, Karolina Pliskova, and Elina Svitolina to lock down their qualifying spots led to tension-filled losses in her last two events in Linz and Moscow. 

"I hope it's going to be a little bit more relaxed from now on," Bertens said during WTA All Access Hour. "I think there is always going to be some stress and tension. You always want to finish your season on good terms, because it's been an incredible season so far.

"Yeah, I think the last few weeks were a little bit more stressful. You try not to think about it, try just to play your match and go from there, but in your head you know like how many points you need to make Singapore. That would be like really a good season ending."

Kiki Bertens posted career-best results at three of the four Slams this season. (Jimmie48)

In the end, Bertens was indeed rewarded for her five consecutive weeks of non-stop play. Not only did she become the first Dutchwoman to debut in the Top 10 since Brenda Schultz McCarthy, but she learned she was the final Singapore qualifier en route to the airport after her opening loss in Moscow. 

"She was trying to get to this place so, so, so hard and she wanted to be here so bad," Sluiter said. 

An incredibly popular and well-liked personality on tour, Sluiter is just as concerned about Bertens' quality of life as he is her quality of tennis. 

"I'm only here for her progress and her progress is, for me, not primarily about results, although tennis is more about results, of course, but it's about her growing as a person and her dealing with tough moments better and better. Because that's going to help her very much in the next couple of years, but it's also going to help her after this. 

"You have to keep that in mind sometimes a little bit, that we are expecting these girls between the age of, well, some even 16 or 17 till 30 to take everything out of their career, where in other career paths, you can keep growing in your job till 40, 50, 60, maybe even 70. These girls all have to do it in this time frame, which can be really tough and really hard on them.

"I like to see this more as a broader time frame. So we're going to take everything out of her tennis career, but we're also going to try and be a stronger person for life afterwards, because that's also going to be something you're going to have to deal with and also not as easy as you think. Probably now when you have so much pressure you think life is going to be great after this, but it's gonna have its own troubles again."

Kiki Bertens and Petra Kviova share a candid moment ahead of the WTA Finals draw ceremony. (Jimmie48)

In a season that has seen Bertens confront her inner demons, she has proven to herself and others that she is up to the challenge. And she has done so by reminding herself that as much as being a professional tennis player is a job, it is also supposed to be fun. 

"The only goal at the beginning of this year was to enjoy my tennis more," Bertens said. "That's what I did. And then of course during the year, there came some goals, like also with the ranking, reaching the top 10, and then Singapore. 

"I have reached all my goals for this year, so I can be really proud of that and happy of that. Hopefully, we can also finish it really well here."