Germany’s Angelique Kerber is unsure how she will fare when she returns to tennis following nearly 18 months out after becoming a mother. Kerber will make her comeback at the United Cup in Australia at the end of the year. “Of course I’m really looking forward to it,” the 35-year-old three-time grand slam winner, whose daughter Liana was born in February, told dpa. “But of course I’m also a bit nervous because I can’t guess how I will perform and what it will be like.” Kerber said the comeback was the “biggest challenge” of her career. “I’m prepared and fit enough to give myself the best chanc…