The 19-year-old Brit appeared to struggle with a blister on her hand throughout the match and required treatment on several occasions from the court doctor.
With the blister affecting her dominant right hand, Raducanu struggled to hit her forehand with her usual power and consistency and the unseeded Kovinic took full advantage to clinch the opening set.
It was an impressive start for the Montenegrin, who had never made it past the second round of a grand slam in 17 singles and doubles main draw appearances.
However, Raducanu was able to receive further treatment to her right hand during the interval and came out in the second set looking more like the player that stunned the tennis world by winning the US Open in September.
She regained control with her powerful baseline ground strokes and leveled the match after another hard-fought set.
The opening game of the deciding set felt like a pivotal moment in the match, with Raducanu agonizingly missing a cross-court forehand that would have given her a crucial early break.
However, the 27-year-old Kovinic showed remarkable poise to eventually hold serve with an inch-perfect smash.
Blistered and bandaged — and often still with a grin on her face — Raducanu continued to battle through the decider with her forehand barely available to her, but in the end that determination wasn’t enough to get the better of Kovinic, who becomes the first player representing Montenegro to reach the third round of a grand slam.
“I really enjoyed it out there,” Kovinic said in her on court interview. “It was a nice experience to play Emma and make it to the third round of a grand slam for the first time after many years.”
“She [Raducanu] had an amazing result winning a grand slam at such a young age. Such a talent and it was a really nice experience. I’m glad that I showed I can play at a high level.”