(Reuters) - Towering Croat Ivo Karlovic announced his retirement from tennis at the age of 44 on Wednesday, bringing the curtain down on a career that saw him fire more than 13,000 aces past bewildered opponents.

Standing 2.11m tall, Karlovic's huge serves and unorthodox style caused problems for some of the sport's biggest names and he won two of his three meetings with world number one Novak Djokovic.