No limits for Serena - Navratilova

Unstoppable Serena Williams could eclipse Steffi Graf as the most successful Grand Slam champion in the Open era, tennis legend Martina Navratilova said on Tuesday.

Navratilova said she expects her total of 18 Grand Slam titles will be tied by the American at the Australian Open starting in Melbourne this month, and that Williams will go on to better Graf's record of 22.

"If she can stay healthy there is no doubt she can go into the 20s. The sky is the limit," Navratilova said in Adelaide.