Qualifier Kovalik, ranked outside top-100, surprised Cilic with his retrieving ability and pulled off the upset win in a nerve-wracking three setter.
"It's first match of the season, obviously you are a bit rusty, without match play. In some situations I was hesitating to go for my shots and he was making more such shots till the end," Cilic, the 2014 US Open champion, said after the match.
Asked if it was a factor that he had not seen Kovalik's game before, Cilic said, "That was the difficulty absolutely".
"I have never played with him, never practiced with. He broke through the Tour last year but I had not seen him play. He played extremely well today, I was not expecting that coming from him."
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"Maybe just to be little more aggressive on returns game. On his service game, when I was returning I was keeping the ball in play while he had the opportunity to go for his shots. Rest of the game was good. I can't say I was not feeling my serve, my forehand, my backhands or movement. Just felt that I was setting up the points while returning was causing me the problem," he said.
Putting up a brave front, Cilic said this defeat does not affect his preparation for Australian Open but he wanted to play a couple of matches before going into the first Grand Slam of the year.
Since winning his maiden Grand Slam trophy, Cilic has not been able to add another Major trophy to his collection. Cilic said he requires to be mentally tough to repeat his US Open success.
"I was happy with my last six months (2016), you have to be consistent at good level. That's the key part to get big results at Grand Slams, where you play for two weeks, five sets. It's difficult mentally to be consistent."
Last year, Cilic lost three big matches, including the fourth rubber of the Davis Cup final against Argentina's Juan Martin Del Potro, when he was up by two sets to love but lost them.
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