No limits for unstoppable Serena Williams, says Martina Navratilova

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

Navratilova said she expects the American to equal her total of 18 Grand Slam singles titles at the Australian Open starting in Melbourne this month and go on to better Graf's record of 22. (Serena takes wrecking ball to rivals)

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

World number one Williams has so far won 17 Grand Slam titles. Only Navratilova and Chris Evert (both with 18) and Graf have won more majors in the Open era.

Navratilova said she was in awe of the consistent excellence of the 32-year-old, who lost just four of 82 singles matches last year.

"I know how hard it is once you get to 30, all those little niggling injuries get worse and it's much easier to have an off day," Navratilova said. "Yet she seems to be playing her best tennis right now."

Williams is the favourite for Melbourne after beating rival Victoria Azarenka 6-4, 7-5 in the final of the Brisbane International on Saturday.

Navratilova, playing in the exhibition World Tennis Challenge this week in Adelaide, said Williams was showing no sign of decline.

"At age 30, she had played about half the tournaments than I had played at 30, so she is fresh in tennis terms," she said.

"But still, she is in her 30s... she is eager and winning is very contagious. Once you get really used to it, you don't want to let go of it."

Former world number one Tracy Austin, also in Adelaide for the World Tennis Challenge, said Williams appeared motivated by surpassing Graf's record.

"I love the fact that she's 32 years old and still seems to be as hungry, if not hungrier, than she has ever been," Austin said. "At 32, and realising that she is towards possibly the end of her career, she is now playing for history.

"Serena likes to be the best at everything and I think she wants to be the greatest player of all time on the women's side."