Hewitt collected his first ATP title since 2010 when he beat an out-of-sorts Federer in Brisbane and he's now nine rankings spots ahead of 21-year-old Tomic.
Cash warned it would be years before Australia's promising younger core, headed by Wimbledon junior doubles champions Nick Kyrgios, 18, and Thanasi Kokkinakis, 17, made a genuine impact on the world stage.
"It's a tough sport out there," said Cash, a headline attraction at the novelty World Tennis Challenge in Adelaide starting today.
"[Lleyton] just goes to show how experience goes a long way, and players are around a lot longer these days - you see players hitting their peak mid-20s and even later.
"But we have got a lot of depth. We have got a lot of good young players coming through and I think that is the encouraging thing.
"In my day, if you were a good grass-court attacking player, you could play 20 tournaments on fast courts; if you were a good clay court player, you could play 20 on clay, and you could still get a ranking.
"Now, you have got to play on similar types of surfaces and really work out and develop your game over many years and it just takes time to build your ranking up. Thanasi, Kyrgios - it's going to take a few years to get right up there."
- AAP