Sainz missed the Saudi Arabian Grand Prix a fortnight ago due to appendicitis and recovered just in time from the procedure to travel to Australia and take part in the Melbourne event.
The Spaniard produced a strong qualifying performance to grab second on the grid behind polesitter Max Verstappen which he said was surprising given the quick turnaround from his surgery and the discomfort he still feels behind the wheel.
"It's been a tough couple of weeks, a lot of days in bed waiting to see if I could make it to this weekend," Sainz said after finishing 0.270s behind Verstappen's Red Bull.
"To put it on the front row after leading through qualifying, I was almost not believing it, especially after how tough it's been.
"I'm very happy to be challenging the Red Bulls this weekend. I was a bit rusty at the beginning yesterday but then I got up to speed and I could finally find the pace and feeling good with a car.
"I'm not gonna lie. I'm not in my most comfortable state when I'm driving out there, but I can get it done.
"A lot of discomfort and weird feelings but no pain, so it allows me to push for a flat out."
Sainz says Williams driver Alex Albon, who was struck by appendicitis at the 2022 Italian Grand Prix, warned him about the type of discomfort he would be facing due to the high loads in the cockpit.
"I feel like it's exactly what Alex told me before jumping in the car when he got his appendix removed," Sainz explained.
"With the g-force and everything, on the inside, it just feels like everything is moving more than normal and you need some confidence to brace the core and the body as you're used to do, but you get used to it.
"There is no pain, there is nothing to worry about. It's just a weird feeling that you have to get used to while driving. Especially on this circuit, we're pulling five to six Gs in some of the braking [zones] and corners."
Sainz felt like missing out on qualifying and the race in Jeddah hurt him more, because he felt still a bit behind on learning his 2024 car, especially on the softest tyre compound.
"I'm still learning this new car, I missed a qualifying session in Jeddah and full race, so probably there were things there that I could have learned through qualifying that I could have applied today," he explained.
"The car surprised me in a couple of corners once we started to crank in the flap. It wasn't the cleanest of laps, but as I said, if you would have told me even five days ago, that I could have travelled here, still recovering, and be P2 on the grid and fighting for pole, I would have taken it."
