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 - a result Sainz 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 0.27 seconds 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.
Source: Autosport