The West Australian clinched his and Erebus Motorsport's first Supercars championship title with a race to spare at the Vailo Adelaide 500, before he and Will Brown also secured the teams' title on the final day of the season.
Few knew, though, that Kostecki had been afflicted by tonsillitis in 2023.
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The revelation comes after Brown went under the knife for back surgery between the Bathurst 1000 and Gold Coast 500, and Simona De Silvestro's arrival to Australia for a Bathurst 1000 wildcard start with Dick Johnson Racing was delayed by emergency appendix surgery.
Circumstances meant that Kostecki, though, had to persist through to the end of the Adelaide 500, without medication, as he explained on the Rusty's Garage podcast.
“I've been sick a few times this year and I've been overseas a few times so, I guess, being on a plane for a while doesn't really help either,” he recalled.
“I've been sick a few times and, every time I've got sick, it was close to when a race event was about to start or I was about to go away, so I didn't really have time to get my tonsils out.
“After Gold Coast, I was sick for two or three weeks and leading up the last round, and finally got myself on the mend.
“It's one of those things, you want to take medication for it but you can't because of how the ruling works, so you've just got to try and minimise the risk as much as possible.
“But yeah, off in two days to get my tonsils out – thank god, finally – and hopefully that part of my life's over because that's pretty annoying.
“All I want to do is go and race cars and I can't, so we'll get that out and sort that problem out.”
Brown had squeezed in a discectomy operation between Bathurst and the Gold Coast, after suffering back and leg pain for the bulk of the season.
The surgery means Kostecki hasn't travelled to the United States this weekend as planned, with Will Brown set to sub for him in the World Racing League final event.