How do you follow up a breakout season? That's the question World No.9 Kiki Bertens as she begins her 2019 season. The 27-year-old went from thinking about retirement at the end of 2017 to taking more ownership of her career and putting in a career-best 2018 season. 

After starting the season at No.31, the Dutchwoman tallied more Top 10 wins than anyone on tour to break into the Top 10 and qualify for her first WTA Finals, where she advanced to the semifinals. Bertens transformed herself from a pure clay-court specialist to an all-court threat, winning titles on hardcourt and clay as well as making the quarterfinals at Wimbledon.

It was a heady season for Bertens, which she capped off with her engagement to her boyfriend Remko de Rijke during the off-season. But Bertens' 2018 success has left her wanting more.

"For me I just want to see how far I can go," Bertens told WTA Insider. "Of course it was a great year, but maybe something more is possible. That's how I feel now and that's how I can push myself to maybe go further. 

"Before I didn't think I could reach the top ten, but now that I'm here I think I can go further. Until that moment, I just want to try it."

Sluiter understands the dangerous temptations after a career-best season. As an ATP player, he broke into the Top 50 and eventually hit a career-high No.46 in 2003. Sluiter admits that achieving this ranking milestone set up a drop-off in effort and focus. 

"I think after a year where - I wouldn't call it a breakthrough year - but the best year in her career, it can go two ways," Sluiter told WTA Insider. "You can relax a little bit more, maybe. Some players will do it. When I broke Top 50 I thought, well, this is probably what I can reach. So I took two steps or three steps less. 

"I haven't seen any of that with her. I have to say I was a little bit surprised because she's hungrier than ever and that's great to see."

"The off-season was completely different from the other off-seasons, in the sense that her level physically, mentally and tennis wise is so much higher. To put it bluntly, the other off-seasons we really had to put in the hard work because fitness-wise there was a lot of work to be done. 

"This time there was a much more emphasis on detail because she's so much fitter, stronger and better. She was hitting the ball great after a three-week holidays in two or three days. So then I told her, hey, if you cannot bring the same intensity on a day, let's not play. That was different from the other years where we just grind, grind, grind it. The other ones were more like, just put in the hard work like Rocky in Alaska or wherever he was, just running and doing.

"Now it's more like, okay, left hip, needs some more flexibility. Trying to get a little bit more kilometers on the serve, racket-head acceleration on the forehand a little bit. So the details.

"It's safe to say that she came into the off-season hungrier than in years past, especially stronger mentally. More an athlete that's thinking herself instead of 'tell me what to do.' That's really good because that's all I want for her because then it's more her thing."

Bertens has reason to feel confident heading into Melbourne, where she posted her best result last year in the third round. She has a potentially difficult draw in the first week, opening against Shenzhen Open finalist Alison Riske, to play either Anastasia Pavlyuchenkova or Monica Puig in the second round, with No.20 Anett Kontaveit or a surging Aliaksandra Sasnovich looming in the third round. But Bertens, who is coming off a strong semifinal run in Sydney, is well aware that she has a great opportunity to pick up points from now until the tour turns to clay in April. 

"I think my confidence is higher than last year when I started in Australia," Bertens said. "Last year I prepared really well and I was practicing really well and then I won three games in my first match here. Now I feel more confident on the hardcourts. I know I can play great."