NURNBERG, Germany – Defending champions and No.3 seed Kiki Bertens progressed into the quarterfinals of the Nurnberger Versicherungscup with a 6-7(2), 6-2, 6-4 victory over Mona Barthel.

The German held a 9-3 head-to-head advantage over her higher ranked opponent, but after making a positive start in a rain-delayed encounter was ultimately overpowered by the WTA World No.18.

Bertens came into the match having lost only one set in her last 11 matches in Nurnberg but was given an almighty scare by a player ranked more than 100 players below her at WTA World No.122.

"It was tough out there," she admitted afterwards. "It was tough with movement. I had to play a bit different to my normal game. I had to go more for more shots. Sometimes it worked, sometimes it didn't."

Indeed, the early part of the opening set was dominated by Barthel, despite the fact she gave up a break point on her opening service game. Bertens missed the inside tramline by a fraction on that occasion and soon found herself 3-0 down against an opponent playing with aggression and accuracy.

Though the Dutchwoman was prone to inconsistency in this match, making a scattering of uncharacteristic errors, she hauled the opening set level at 4-4 by holding to love after a run that saw three consecutive breaks.

Two holds followed but Bertens' hitting was becoming more regular, so when Barthel buckled at break point down in the 11th game, delivering an ill-timed double fault, it seemed it may be a critical error. However, the home player broke her opponent back to force a tiebreak in what had been a topsy-turvy set.

The deciding game threatened to be as unpredictable as those that had come before, but the set finished as it had started, with Barthel positive and Bertens erratic. The consequence was the WTA World No.122 striking the opening blow.

Accuracy continued to desert the higher ranked player as she gave up a cheap break in the opening game of the second set, sending a wild forehand wide then submitting with a double fault.

True to the crazy nature of this match, though, she won the next game and then four of the next five to move into a commanding position in the second without ever hitting her best level.

Barthel, though, had come off the level she had found in the formative stages of the match and meekly surrendered the set as she failed to hit her targets with any consistency.

Just as it seemed that Bertens finally had the match where she wanted it, she found herself facing two break points in the opening game of the decider. Although these were saved and she broke in the following game, Bathel showed excellent mental fortitude to turn the set around and move 4-2 up, notably breaking with a super backhand down the line.

Bertens, who was increasingly frustrated, netted a simple volley when she had a chance to break back in the eighth game, but got the game when Barthel made two more slack errors.

That proved the opening the Top 20 player required, as she reeled off the next couple of games to win a tense encounter and set up a match against Kirsten Flipkens as she continues her quest for a third successive Nurnberg title.