The 57-year-old Navratilova who won 18 grand slam singles titles, and 41 in women's and mixed doubles, is also confident the 32-year-old Williams will equal her singles haul at the Australian Open, which begins next week in Melbourne.
"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 where she will play in the World Tennis Challenge exhibition later on Tuesday.
"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.
"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."
Australia's Margaret Court holds the overall record of 24 grand slam singles titles, while Germany's Steffi Graf won 22 titles in the Open era.
Williams is widely considered the best player of her generation and has been virtually unchallenged in the modern era, with her powerful game proving difficult to defuse for many counterparts on the WTA tour.
The American, however, has also been hampered by a number of serious injuries, including a knee injury in 2010 and cut foot that forced her to miss almost a year on the tennis circuit in 2010-2011.
Paradoxically it was those injury-enforced layoffs that could be helpful in prolonging her career, Navratilova suggested.
"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.
"She seems to be playing her best tennis right now."
Former world number one Tracy Austin echoed Navratilova's sentiments and said she felt Williams, who beat rivals Maria Sharapova and Victoria Azarenka to win last week's Brisbane International, was being motivated by the record books.
Williams poignantly saluted compatriot Billie Jean King when she won her fifth, and latest, Australian Open title in 2010 to join King on 12 grand slam titles saying at the time that she had told King 'I tied you' after her victory over Justine Henin.
"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."