Qualifier Kovalik, winner of just two Challenger titles in his career, was up against a Grand Slam winner but made Cilic's life as tough as possible with his never-say-die spirit and in the end was a proud winner.
Kovalik, ranked 117, showed nerves of steel in a hard-earned 7-6(5) 5-7 7-5 win over the 'rusty' top seed.
Cilic, who ended the 2016 season with a career-best rank of six, was pushed by the Slovakian in two hours and 48 minute match and was rewarded with the biggest win of his career.
Kovalik next faces Daniil Medvedev, who shocked eighth seed Yen-Hsun Lu 6-4 6-3 in his second round.
It is first time since 2010 that a top seed has lost his first match after Robin Soderling had lost to Robby Ginepri.
The Slovak said it was hard to believe that he has won against a top guy but it gives him confidence that he can match them.
Asked when he thought that he could beat Cilic, Kovalik said with a big smile, "The last moment when I beat him."
"I tried to win each point. I tried to move him and was focused on the job."
Cilic looked a bit rusty at the start as it took him almost 10 minutes to hold the first service game in which he saved a breakpoint and fought off five deuce points. He eventually sealed it with an ace.
Kovalik impressed with his ground-strokes, specially backhand, retrieving ability and the court coverage. He managed to stretch points by engaging Cilic in rallies.
The Slovakian too faced two early breakpoints when he hit a double-handed backhand long, but saved both, first with a backhand winner and next with a forehand winner.
Disclaimer: No Business Standard Journalist was involved in creation of this content
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