It was no cakewalk for birthday girl Angelique Kerber in the second round of the Australian Open, with the women's top seed and defending champion dropping a set against world No. 89 Carina Witthoeft. Kerber celebrated her 29th birthday with a shaky 6-2, 6-7 (7-3), 6-2 win over her fellow German. But the world No. 1 still has plenty to do if she wants to enjoy another round of celebrations at Melbourne Park. Her title defence continues tomorrow against Czech Kristyna Pliskova, the twin sister of fifth seed Karolina.