It was a moment that cut to the heart of what Invictus means.

A pause in play unique to these Games. One that reminded everyone present of the emotional barriers that competitors here must overcome on a daily basis.

Wheelchair tennis player Paul Guest was comforted by his Dutch playing partner Edwin Vermetten at Sydney Olympic Park Tennis Centre, after the sound of a helicopter caused Guest's PTSD to resurface, momentarily leaving him unable to take his serve.

"I get all hot, I still feel the burning," explained Guest, a mine warfare specialist injured in 1987.

"I can still feel everything and smell everything. That very split second, it all comes back in my head."

Paul Guest serves during his match on the final day of the wheelchair tennis event at the 2018 Invictus Games. ( ABC News: Taryn Southcombe )

Vermetten quickly assessed the distress of his playing partner.

"The guys got around me," Guest said.

"[We were] just singing a song from Frozen — Let It Go. We were singing it together just taking my mind off it talking rubbish in my ears and that's what it is all about."

Invictus Games competitor Paul Guest with his wife Michelle outside the wheelchair tennis venue. ( ABC News: Taryn Southcombe )

That was on Monday. On Tuesday the popular Disney song got a special one-off run from the loud speakers during a regular break in play. The 54-year-old's UK teammates on the hill sung on enthusiastically.

Guest and Vermetten then powered to another win.

The spirit of Invictus

The pair have formed a successful combination in the competition, despite only meeting in person for the first time a couple of hours before their first match.

"He is an inspiration on court," Guest said.

"It was like I had known him all my life, he is just one of those guys that you bond with straight away.

"The whole spirit of Invictus was in Edwin."

Vermetten, competing in his third Games, says his response came naturally.

"We heard a chopper coming over and we had earlier, saw the signs," he told the ABC.

"[Guest] made some movements with his head and I immediately noticed he needed some care.

"I said look in my eyes, look in my eyes, and he did.

Paul Guest and Edwin Vermetten share a fist bump on court on the way to their emotional victory. ( Getty Images: Invictus Games Sydney 2018 )

"I said: 'you know the Frozen song, Let It Go? He was looking at me really surprised and I said, 'let's sing together'. And he let go. It was really amazing."

Guest bounced back quickly despite the fact that he's playing with a new teammate in a sport that he's not experienced in.

The pair continued on their winning ways on Tuesday.

Finding purpose again

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In the interview room after the match, Guest explained how Invictus has given him a new purpose.

"It doesn't matter where I come. I haven't set my heart on anything, the only thing that matters is as long as I have done better than yesterday, I have won," he said.

"Last year, winning a Bronze in Toronto [at the previous Invictus Games] was great, but the biggest thing for me was when I came home and my six-year-old's teacher called me and said my son drew a picture with me in it."

Guests' wife and carer Michelle, by her husband's side during the interview, says her partner has made a remarkable comeback.

"There was a point where he just wouldn't come out of the bedroom, let alone the house," she said.

"I am really pleased, so pleased. That he is getting on and doing something, finally..."

"I just felt so useless," chimed in Guest.

"I was ashamed of what I had become. I was serving, I was supporting my family and then I was sat there, I was nothing.

"I was just paralysed, and my kids had to lift me in and out of the bath."

Guest has come a long way since and knows he now has an important role to play helping to rehabilitate others, standing on the shoulders of those who are supporting him.

"I enjoy the camaraderie, I just enjoy meeting people," he said.

"The thing is, when we are here, it's one of things, you don't go up to somebody and say, 'what happened to you then? How did that happen?' Nobody cares.

"You are here, it doesn't matter.

"These guys don't see disabilities, they just make us feel welcome all the time."