CHARLESTON, SC, USA - Defending Volvo Car Open champion Kiki Bertens may have transformed herself into an all-court threat over the last 12 months, but the World No.6 is never happier than when the clay season begins. The Dutchwoman's phenomenal 2018 campaign gained traction right here in Charleston last year, and Bertens says she's even more relaxed this year as her title defense kicks off.

"For me, it's more freedom on the clay," Bertens told reporters at All Access Hour. "I can do a lot more. So for me, it's easier to change to clay then back to the hardcourts again."

After winning her first Premier title in Charleston last spring, the 27-year-old tore through the remainder of the season. In addition to making the biggest final of her career a month later at the Madrid Open, the self-described clay-court specialist finally found consistent footing on all surfaces. 

It was a season of breakthroughs for Bertens, as she made her first quarterfinal at Wimbledon, won the biggest title of her career in Cincinnati, and went on to qualify for her first WTA Finals in singles, where she advanced to the semifinals. 

Much to her own relief, Bertens carried that momentum into 2019. She won her first title of the season at St. Petersburg and posted back-to-back Round of 16 showings in Indian Wells and Miami, the best Sunshine Double results of her career. Through the early hardcourt season, Bertens ranks No.2 on tour in Aces (153, behind Karolina Pliskova), 1st Serve Points Won (72.3%, behind Ashleigh Barty), and Service Points Won (62.9%, behind Barty).

As a result of her solid start, Bertens comes into her favorite part of the season already sitting at No.10 in the Porsche Race to Shenzhen. 

"It's different than the last few years, for sure," Bertens said. "Then, I knew if I did not do well in the clay court season then I would drop out of the Top 50 or something. Now that's not going to happen so I feel a little bit more freedom. 

"But still, a lot of things could happen, you never know. Last year I was quite relaxed still coming here because I knew it was a long clay season and I played well here. So we'll see."

Kiki Bertens practices with 2018 runner-up Julia Goerges ahead of the Volvo Car Open. (@Jimmie48)

Bertens' prowess on clay has been well-proven, and how she handles the upcoming stretch through the French Open will be one of the important storylines of the spring. Since the start of the 2016 season, Bertens has averaged 75.6% of her service games won on clay and 43.0% of her return games won on clay. Over that same time period, she is averaging 68.4%/39.0% on hardcourt and 69.3%/35.9% on grass. 

"I'm much more happy again," Bertens said. "Already, the last few days of practice I've really wanted to go out there and practice. The sliding I think the first few days is difficult, the spin balls, getting used to that again."

"I grew up on the green clay. The first few years it was red clay but then I practiced for like 10 years on the green clay, also indoors. So I was pretty used to it."

Among active players who have played 30 or more matches on clay, Bertens ranks 5th in career winning percentage at 69.3%, behind only Serena Williams (83.3%), Maria Sharapova (81.5%), Elise Mertens (78.4), Venus Williams (73.5%) and Simona Halep (72.2%). Isolating Bertens' clay results to the start of 2016 to now, her winning percentage jumps to 77.1%. In fact, since the start of the 2015 season, no one has won more matches on clay. 

Bertens begins her title defense against Italian qualifier Martina Trevisan on Tuesday.