LONDON - Eighteen-times grand slam singles champion Martina Navratilova has quit as a part-time consultant to world number nine Agnieszka Radwanska after just five months in the role. The Pole hired Navratilova in December with the aim of reaching the latter stages of more big tournaments but the American great said she had been unable to dedicate enough time to the job. "I am stepping down as Agnieszka's part-time coach. I think I underestimated the time it would take to make this a good and proper situation for both Agnieszka and me," the 58-year-old explained in a statement. Radwanska was beaten in the 2012 Wimbledon final and is still searching for her first grand slam title. She was beaten in the first round of the Stuttgart Grand Prix this week by Italy's Sara Errani.