I've posted this before, but...
I think it is hard for people who never watched Cash play to understand how unbelievably injury-riddled he was, and how much it constrained his massive potential. He had been a great junior player - ranked number 1 in the world, sweeping all before him. In 1982 he won the boys’ singles and doubles titles at both Wimbledon and the USO, and the doubles title at Roland Garros for good measure.
He really stormed onto the pro scene in late 1983, at 18 becoming the youngest ever to play a Davis Cup final (in a time when that really meant something) and winning a live fifth rubber to take the title for Australia against Sweden. In 1984 his rise was meteoric, blitzing top seeds like Wilander, Curren and Gomez at Wimbledon before running into McEnroe in the semi-finals. He then backed it up with another semi-final at Flushing Meadows, again brushing aside Wilander and pushing Lendl to the brink in a fifth set tiebreak.
All this boosted his ranking to #8 in the world whilst he was still a teenager, and everyone was expecting huge things in 1985. Unfortunately he was already struggling with a recurrence of an old back injury which severely damaged his results for the first six months of the year, eventually forcing him off the circuit after Wimbledon 1985 for a full nine months.
In March 1986 he finally returned to tennis on an extremely limited basis - barely walking, zero fitness. By the time Wimbledon 1986 rolled around he had played five matches in 12 months (none on grass) and was ranked 413. Then, three weeks before Wimbledon started, his appendix burst and required emergency keyhole surgery. Despite all this, he accepted a wildcard to play - and made the quarterfinals. He beat #18 Vilas and #2 Wilander on the way, finally losing in four tough sets to seventh seed Henri Leconte - outgassed more than outplayed.
The rest of 1986 was gradually rebuilding fitness, culminating in leading Australia to another Davis Cup title - again over Sweden. He beat #5 Edberg in the first rubber, and then in a live fifth rubber he came back from two sets to love down to beat #11 Pernfors in five sets. It was absolutely glorious.
That Davis Cup win proved to be the catalyst for a great 1987. The Australian Open final - five set loss to Edberg, after beating #4 Noah and #1 Lendl in the QF and SF respectively. Semi-finals at Queens, this time beating Edberg. The Wimbledon title, well known - Forget, Wilander, Connors, Lendl. Another final in Sydney, beating Becker indoors and losing a tight match to Lendl. The 1988 Australian Open - another win over #1 Lendl in the SFs, on hardcourt this time, and one of the great Slam finals against Wilander, losing 6-8 in the fifth. 1988 Wimbledon - another QF, only losing to a rampant Becker.
Then back niggles set in and, in April 1989, he ruptured his achilles. It was a devastating injury for anyone, doubly so in those days and trebly so for an explosive and physical S&V player like Cash. It took him off the circuit for a full year, and effectively spelled the end of his career as contender. He was only 23.
I speak without a word of hyperbole when I say that I think we as tennis fans were robbed of a genuinely great fast-court player in Cash. During his brief runs without injury he defeated a host of genuine greats of the game, often on their favourite surfaces. How well he could have done with five or ten straight years of fitness, I am not sure. Certainly he would have won the Australian Open, and he probably should have won the US Open as well. I would wager he would have given Becker and Edberg plenty to worry about at Wimbledon too.