Cash said a group of junior players looked promising and could help Australia revive its former glories on the international circuit.
"Expect Australian tennis to be quiet in the coming few years but some good juniors are coming through now and in the next four to five years you should expect an Australian tennis boom," Cash told Reuters in an interview on Wednesday.
For the last decade Hewitt and Philippoussis shouldered the hopes of a nation that had dominated the tour during the 1960's and early 70s.
Over the last 25 years, however, Australians have won only five men's grand slam titles.
Hewitt held the top ranking for a total of 80 weeks from 2001 to 2003 but has not won a major title in almost five years and has slipped to outside the world's top 20.
"Hewitt has had a bad run with injuries. But if he wanted to come back to the top three he could. It is difficult to win grand slams with (Roger) Federer and (Rafael) Nadal around."
Philippoussis, who hopes to return to the tour full-time after several injury-plagued seasons, was set to play the over-30s event organised in Athens this week.
But another injury he sustained earlier in the year forced him to pull out.
The 30-year-old reached a career-high eighth in the rankings in 1999, the year after he lost to fellow Australian Pat Rafter in the U.S. Open final. He reached the Wimbledon final in 2003, losing to Federer.
"For Mark it is hard to come back when you keep getting injured," said Cash, who coached Philippoussis in the past.
"To be competitive on the tour you need a good year of match play. If he doesn't get that, his chances (of coming back) are slim."
