Borna Coric said returning from a knee operation was tougher than expected as he sets his sights on a strong finish to the season.
The Croatian, 20, underwent surgery in September last year and has endured a mixed 2017.
Coric claimed his maiden ATP World Tour title by winning the Grand Prix Hassan II in Marrakech in April, but holds a 14-16 win-loss record this year.
The world number 44, who is the sixth seed at this week's Croatia Open, said the injury had continued to trouble him to begin the campaign.
"It was harder than expected," Coric told Omnisport.
"I was struggling a lot during the first three months of the season because it was still a bit painful and obviously I couldn't play my best tennis."
Coric saved five match points in the Marrakech final against Philipp Kohlschreiber and sits third in the Race to Milan - the Next Generation ATP Finals.
Along with his maiden title, Coric has also beaten world number one Andy Murray in Madrid and Austrian Dominic Thiem in Miami this year.
"I think I had a good season so far with some very big results," he said.
"But I also had some losses which could have been avoided."
Having missed the back end of last year due to his surgery, Coric - a former junior world number one - has a chance to boost his ranking late in 2017.
However, his focus is on his fitness, saying: "My main goal is to stay healthy, that is the most important. And of course to strengthen my game and play better.
"I have been playing good so far but not as well as I want so I definitely need to improve my level of tennis."