If anyone understands the pain of injury then it is Mackenzie McDonald. 

The American stunned a hobbled Rafael Nadal under the lights of Melbourne's Rod Laver Arena in the biggest win of his career. 

World No 2 Nadal described himself as mentally destroyed as his title defense ended in round two. His wife, up in the stands, was in tears as his body broke down. 

Rewind back to 2019 and it was McDonald, just three games into a first round doubles match at the French Open with fellow American Reilly Opelka, that saw everything change. 

McDonald ripped his right proximal hamstring tendon and after going under the knife for surgery, he spent two months barely able to walk.

'It took a really serious team and a lot of hard work,' Mackenzie said, via the ATP Tour. 

'The surgery was pretty big. I have a massive scar underneath my butt and I wasn't able to walk for the first month-and-a-half, two months. Time was going by really slowly.'

McDonald was cooped up in his Floridian apartment unable to leave - or even make it to his bathroom to shower. It was down to his grandma to lift his spirits as she regularly sent flowers to cheer him up.  

'I didn't leave my apartment. I was barely able to shower, go to the bathroom. I couldn't walk.' 

McDonald, now 27, is far from an overnight success. Beating a former world No 1 has been 24 years in the making since his first coach, Rosie Bareis, sat on a milk carton at the Harbor Bay Club in Northern California to toss balls up to him as a three-year-old. 

Son of Michael McDonald, a self-made Scottish-English oral surgeon, and Vivian Young, of Chinese heritage, the big-striking McDonald was guided down the tennis path from an early age. Training at 6.30am before school became the norm. 

'I remember in first grade with Rosie,' he previously explained. 'It was a lot of hours. She would sit on a milk carton and drop balls. She was tough on me.' 

Raised in the Bay Area, McDonald attended Piedmont High School in California, the same high school which once honed the skills of former world No 4 Brad Gilbert.

Every now and then McDonald would raise an eyebrow on the juniors circuit but it was not until he walked on at UCLA as a freshman in 2014, listed as the No. 1 player coming into college, that his notoriety began to peak. 

The McDonald family quickly became known around campus with Mackenzie thriving on the tennis court and his older sister Dana a member of the UCLA gymnastics team - that would eventually go on to make the junior Olympic team.

McDonald watched on with some envy as people in his age bracket began to make waves on the ATP Tour, all while he was balancing his training with his political science major. 

But sticking at it for three years, in 2016 McDonald became the first college player since University of Georgia's Matias Boeker in 2001 to win both the singles and doubles NCAA titles. 

He was a class apart - and months after winning those titles he announced he was not returning to UCLA for his senior year. McDonald was taking the plunge to go pro. 

Within weeks he found himself hitting with Roger Federer, one of his sporting idols, and come 2017 McDonald was making a splash at Wimbledon, eventually losing out in round four to Milos Raonic of Canada. 

Close friends with fellow American players Taylor Fritz, Frances Tiafoe and Tommy Paul, McDonald knew he needed more to his game to get over the top. 

Kobe Bryant, along with Federer, is one of McDonald's biggest inspirations and in 2019, a couple of months after finally being able to walk again, he got to meet the LA Lakers legend. 

They shook hands, chatted and Bryant wrote a message of support in the front of his then-new book Legacy and the Queen, which he wrote with Annie Matthew. 

McDonald simply labelled the Instagram post: 'Mentality'. 

It was that mentality that has stuck with McDonald since he returned to action in 2020 - going on to be fitter than ever before after the surgery which threatened his career on the ATP Tour. 

The now-Orlando resident, who is dating fellow US player Maria Mateas and is coached by former world No 15 Robby Ginepri, knows his strengths and knows Bryant never felt inferior looking into the eyes of his opponent. 

Before his Melbourne showdown with Nadal, McDonald sought the advice of close friend Tiafoe. 

It was something of a running joke when the draw aligned that McDonald was the only one of four friends not to have defeated the Spaniard. 

'I told Mackie, "Listen, you could join the three amigos: me, Fritz, and Tommy. We all got him. I don't know if you want to join it",' Tiafoe said, as per the ATP. 

'Also, I told him, "You're going to be in a position to win today. You can win today."'

McDonald, who entered the tournament with a singles ranking of No 65, laughed, not taking the size of the task lightly.

It was not until he returned to the locker room after a 6-4, 6-4, 7-5 win that the size of his accomplishment hit home. While emotion washed over him his dad Michael was posting to Instagram photos of him as a child playing tennis. This was a win for his entire family's belief in the process. 

'I was in the locker room, and I was, like, "Hey, that's actually really big for me", because I haven't beaten someone of that calibre,' McDonald said. 

'GOAT wins don't come easy,' Tiafoe added. 'Something to tell his grandkids one day, and you have to be happy for that guy.' 

Tiafoe is right and this will be a story McDonald will never tire of telling his nephew Rome. 

It's Yoshihito Nishioka up next in round three having beaten Nadal but it was overcoming his inability to walk that remains his biggest victory of all.