Passing off the injury as a bruise that would disappear, Chambers wasn't worried. Then eight hours later, her leg collapsed from under her.
The severe blow to Chambers' shin had caused one of four internal compartments in the leg to swell. The leg muscles were wrapped in a layer of fibrous tissue with no give, so the pressure caused by the swelling couldn't subside. The nerves were cut off from all blood flow and began to die. Chambers was in trouble.
In hospital, a surgeon told her she had come within 30 minutes of facing amputation. Part of the leg had died. As doctors battled to save the leg, they were uncertain what functionality would return.
Surgery meant slicing open her calf lengthways on both sides to relieve the pressure, leaving gaping wounds that needed major skin grafts taken from her thigh to help the heeling.
Doped up on morphine, Chambers remembers hearing the news she could be permanently disabled.
"But I still remember at that moment I knew I wasn't going to accept that. I didn't want to be disabled."
Almost two years of fulltime physical therapy and four surgeries later, Chambers still walked with a limp and couldn't travel far on foot. She was told she would not be able to run again.
As a self-confessed gym rat, an accomplished dancer and a rower at Berkeley, Chambers thought she'd give swimming a go.
"I thought in the very least that swimming might be freeing," she says.
Soon, Chambers wasn't letting herself out of the pool unless she'd completed 80 lengths. Friends invited her for a swim in San Francisco Bay. The open water and the chill - the temperature can sink as low as 8C - became addictive. "You get out and you feel like you've conquered the world," she says.
Chambers' swimming gradually escalated in seriousness. Friends encouraged her to complete the Escape From Alcatraz triathlon early in 2010: the course begins with a 1.6km swim from Alcatraz to San Francisco city and ends with a mountainous 13km run.
"I didn't even think about it at first. I didn't want to set myself up for disappointment. But my friends got me to try it and I did a practice run. I wasn't fast but I could run," Chambers says.
Her leg held up fine. She came in towards the back of the pack, but the results were inconsequential.
"People told me I couldn't do it," Chambers says, grinning just at the memory of the achievement. Later that year she entered a 26km run, two laps of that same mountainous course, competing in pouring rain.
She finished hours after other competitors but a professional photographer captured a grinning Chambers, covered in mud, at the end. "The result didn't matter. I was just having the time of my life."
That conquest drove her to test what she was capable of in the water.
The conversation turns to her first long distance swim in May 2011, a team relay swim from San Francisco to the tiny Farallon Islands, almost 50km off shore.
The small crop of islands is renowned for its shark population and is part of the infamous "Red Triangle", home to a fair chunk of the world's shark attacks. It is here that tourist operators offer cage diving with great whites as an attraction.
"Every stroke I took I thought it's going to be my arms or my legs," she says, laughing.
Two weeks later Chambers gathered an all-women team to do this same swim. She volunteered to take the final leg of the swim in the dark and closest to land, where the sharks like to congregate.
She finished the year with relay swims around Manhattan Island and across the English Channel and then made her first mistake. In a fit of bravado, Chambers attempted to do a spontaneous solo crossing of the channel. She was ill prepared and had not trained. "I got pulled half way. I was just a mess." Disappointed, she vowed she would be back.
Last year, she began concentrating on solo conquests and this time she was prepared. While visiting her parents in Te Kuiti that March, Chambers took on the Cook Strait, swimming from the South Island to Wellington.
She survived 6m swells during the 8-hour swim. Shortly afterwards she had trouble breathing and was admitted to Wellington Hospital for oxygen and rehydration.
"The doctors looked at me so strangely when I told them I'd just swum Cook Strait. It was like they were giving me a pysch evaluation."
Chambers visits New Zealand whenever she can, returning four times last year alone. She still considers herself a proud Kiwi and swims in a skullcap emblazoned with the New Zealand flag.
Eventually, she will move home. But not before she has conquered a few more stretches of open water.
In August last year she swam 35km across Lake Tahoe, a swim made more gruelling by the lack of buoyancy in fresh water.
In September, Chambers was part of a team that attempted the world's longest-ever relay swim of 545km down the California coast, raising $1.2 million for the Semper Fi fund that assists injured marines.
The relay swim was halted after six days and more than 300km because of incessant attacks from jellyfish. Chambers' stomach looked like she had the measles, she says.
In November she swam 42km across Hawaii's Molokai Channel to Oahu, waters infested with tiger sharks and Portuguese man-of-war jellyfish. She was stung 50 times during the nearly 20 hours she was in the water.
The rapidity of Chambers' progression belies the raw, brutal physicality of it all. The salt water makes her lips and tongue swell.
She swims with a support boat nearby and a skipper yelling out to give her the right level of tough love to keep her going.
There is a kayaker also, a marker to keep her swimming in the right direction. She coats her body in engine oil in an effort to stop the chafing that can add to the agony.
The sheer physical and mental strain of the swim usually brings Chambers to tears while she swims. Adrenaline shields her from much of the pain, but her body is useless in the days afterwards.
Numb with adrenaline, she cannot remember those victorious times - the moment she reaches shore.
She has to gain weight before each major swim to help insulate her from the cold but even so has to warm up slowly to make sure she doesn't pass out as blood returns to the extremities.
"I don't know how I do it," Chambers says. "But when I'm out there I feel so alive. That sense of purpose you feel on the swim is amazing."
The completely unrestrained way Chambers has jumped head first into this world, and achieved remarkable success, has annoyed some open-water swimmers.
Now ranked number 25 in the world of open-water swimming, in less than two years she has mastered distances people have trained a lifetime for: "I get a lot of flak from people. Who are you? What are you doing?"
As always, Chambers is looking ahead to her next big open-water swim. She has lined up a crossing of the English Channel for September and has two more swims planned that she says are a secret.
She has the famed Oceans Seven in her sights, a collection of the most difficult seven swims from all seven continents of the world.
She has already completed two - Cook Strait and the Molokai Channel.
Only one person in the world, Irishman Stephen Redmond, has done all seven. If Chambers has her way, she will be the first woman.
Before her English Channel crossing, Chambers will swim four or five times a week for several hours in a pool 20 minutes north of San Francisco as well as the bay each day.
Her American boyfriend, Joe Locke, whom she met through swimming, keeps her on-task. "He's an amazing support network. He gets how tough this is," she says.
Chambers knows eventually age will get the better of her and she'll have to stop. Her mother worries constantly about her on the long swims and has hinted more than once about the possibility of grandchildren.
"I don't know yet whether that request will be granted,"she laughs. "We'll see."
So far Chambers has done her open-water swims without the help of sponsors, covering her expenses from what she earns in her day job.
Non-swimmers don't quite get it. "Once you start to say that you swim without a wetsuit and the size of the stretches of water, people walk wide circles around you," she says.
After hours swimming in cold, shark-infested water, Chambers has come to terms with the risks she faces. She tries not to think about it too much and, typically, turns the subject into a joke.
"I read a joke on Facebook the other day that said, 'If someone came into my house in the middle of the night just wearing a Speedo, I'd kill him too'."
Her new life exhilarates her.
"Life on land is so structured and so safe. I like having this other side of things where I roll the dice. I wasn't living before I injured my leg."
