Rune was a few games from exiting the tournament and had difficulties dragging himself around the court when he was behind 2-1 in sets and trailing 2-4 in the fourth set.
Still, the exhausted Danish tennis player came back in the match in the third round of the Australian Open against Kecmanovic and won 6-7, 6-3, 4-6, 6-4, 6-4 after more than three hours of play.
Now awaits what many had previously identified as Rune's biggest challenge in the year's first grand slam tournament.
In the fourth round, the 21-year-old Dane faces world number one Sinner. The two have met four times and won two each.
Rune explained that he was not feeling well in the last two sets of the match.
"It was pure willpower. I really didn't feel well in my body. My legs hurt from the start of the fourth set, and at the end of the match it was pure willpower", said Holger Rune after the match.
"I gave everything I had in me. It wasn't the wildest legwork I had today, but I gave it what I had", said Rune who struggled with cramps in the fourth set.