So when last night, barely five minutes into yet another epic trans-Tasman netball match at Christchurch's Westpac Arena and with Australia hot on attack, she doubled over and desperately signalled for time, things didn't look good.
When she straightened up, still clutching her head, a spectacular amount of blood was seeping through her fingers -- a sight more common on a rugby field than a netball court.
A stray elbow from rookie Australian goal shooter Kate Beveridge had caught Williams square above the eye as the pair jostled for position in the confined spaces of the shooting circle. The skin split, the blood poured and the crowed murmured anxiously.
Silver Ferns medical staff took the maximum two minutes allowed to treat the injury, pushing it out as far as possible to stem the blood and patch the wound with a huge dressing.
With replacement keeper Leana de Bruin warming up on the sidelines, and the umpires looking pointedly at their watches, she took the court again.
Dazed Williams might have been, but she certainly wasn't confused and took very little time to hit her straps again, snatching rebounds, hustling shooters and hunting intercepts with her customary careless disregard for personal safety.
New Zealand couldn't quite come up with the goods for Williams, falling short in a desperate final quarter to lose 51-53 after leading 28-27 at halftime.
No grudges were held, however, and although Williams wasn't available for after-match comment, New Zealand coach Ruth Aitken was quick to point out that the incident was just part and parcel of international netball.
"It was just one of those random things. When you get a lot of people running around in a very small space, things like that do happen," Aitken said.
"It just goes with the territory -- I was very pleased we were able to patch Casey back up again and keep her going, even if she did look a bit like a pirate..."
Australian coach Norma Plummer was equally philosophical about the injury.
"I didn't see the clash -- I believe Kate went back then came forward...She must have got Casey in just the right spot -- I thought she looked quite stressed, and it looked quite nasty. I think it shook her up for a while but these things happen."
Williams, 23, has become an indispensable part of the Silver Ferns so quickly that it's difficult to remember exactly who held the position before her, although Vilimaina Davu retired only recently.
Silver Ferns assistant coach Wai Taumaunu, one of the New Zealand netball's most uncompromising defenders, is quite definite about Williams' ability, saying simply: "Casey is one of the best defenders I've ever coached."
So when the teams line up for the second test in Auckland on Saturday, there's no doubt at all Williams will be on court, perhaps with a stitch or two and an extra bandage, putting her body on the line. Again.
