Sixth seed Kiki Bertens eased into the semi-finals at the Stuttgart Grand Prix on Friday, blasting past two-time champion Angelique Kerber 6-3, 6-4.The Dutchwoman's powerful serving ensured that she did not face a single break point during the match, and her big hitting saw her notch up 29 winners. In contrast, Kerber's struggled for consistency on her serve and a single break in each set was enough to seal her fate.