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."
|Red Sox ace Sale reaches 300-strikeout milestone|
|Neville: Rashford as good as Mbappe and Dembele|
|MLS Review: Montreal win derby, Atlanta put four past Galaxy|
|Rahm proud of consistency in 2017|
|Madrid can overturn deficit as captain Ramos remains calm|
|Neymar wouldn´t have treated Messi like he did Cavani – Forlan|
|Twins´ Dozier calls for mandatory protective netting after young fan hit by foul ball|
|Bengals boss Lewis warns players about their job security|
|76ers set no limits on Simmons|
|Zidane not worried after Madrid lose ground on Barca|
|Casemiro calls for Madrid calm after shock Betis loss|
|Sane sends message to Guardiola as Manchester City shake off West Brom|
|Keep calm and carry on – Allegri backs Higuain to rediscover scoring touch|
|Bosz proud of table-topping Dortmund after Hamburg triumph|
|Marlins slugger Stanton crushes 56th home run|
|Montella points to schedule after uninspired Milan win|
|Conte confirms Luiz will be fit to face Atletico|
|Manchester City boss Guardiola hopeful Gundogan injury not serious|
|Real Madrid 0 Real Betis 1: Ronaldo´s return ruined as Sanabria stuns LaLiga champions|
|Wilshere is ready for the league - Wenger|
|Simeone lauds ´different class´ Griezmann|
|Mourinho praises United´s professionalism in EFL Cup win|
|Spurs to host West Ham in EFL Cup fourth round|
|Froome: Time to put my feet up|
|West Brom 1 Manchester City 2: Sane at the double amid fresh Gundogan fears|
|Manchester United 4 Burton Albion 1: Mourinho´s men begin EFL Cup defence in style|
|Arsenal 1 Doncaster Rovers 0: Walcott hits winner for Wenger´s men|
|AC Milan 2 SPAL 0: Penalty double seals comfortable win for Montella´s side|
|Chelsea 5 Nottingham Forest 1: Hazard on form as Batshuayi claims hat-trick|
|Juventus 1 Fiorentina 0: Mandzukic on target as Bianconeri maintain winning start|
|Returning Gundogan limps out of impressive Manchester City return|
|Hamburg 0 Borussia Dortmund 3: Bosz´s men cruise back to Bundesliga summit|
|Broady stuns Mannarino in St Petersburg|
|Athletic Bilbao 1 Atletico Madrid 2: Correa and Carrasco strike in second-half surge|
|Simon suffers surprise Moselle Open exit as home hopes struggle in Metz|
|South Africa announce India schedule and planned four-day Zimbabwe Test|
|I have nothing to prove - Paulinho|
|Prodl pens new four-year Watford contract|
|Monaco renew Subasic, Raggi deals|
|Stoneman is ready for Ashes test - Cook|
|Sampson sacked as England Women´s boss|
|Manchester United to act against ´offensive behaviour´ after Lukaku race row|
|Atletico Madrid´s Wanda Metropolitano to host 2019 Champions League final|
|Flying Dutchman Dumoulin crowned world champion|
|Wickmayer topples Peng in Guangzhou, Hogenkamp makes Seoul progress|
|Solomona focused on Sale rather than England return|
|‘Raging Bull’ Jake LaMotta dies aged 95|
|Chelsea confident of recruitment compliance amid FIFA investigation|
|Brighton seal permanent signing of Newcastle keeper Krul|
|Louw, Nkosi and Schreuder get Springboks call|
|Perez donates 3m pesos to Mexico earthquake fund|
|Injured Mathews out of opening Pakistan Test|
|Double leg break victim Rossi to attempt incredible Aragon GP return|
|Barcelona condemn Spanish crackdown on Catalonia independence vote|
|Muguruza marches past Puig, Pliskova and Kerber progress in Tokyo|
|Benzema seals Real Madrid stay until 2021|
|Smith defends captaincy ahead of landmark ODI|
|Perisic promised Inter can match Manchester United|
|The race is on - Aubameyang lays down Lewandowski challenge|
|Cristiano Ronaldo´s Real Madrid return confirmed|
|Bellew tells Haye ´I oblige you´|
|Mbappe can prove he´s ready for Madrid at PSG - Morientes|
|Five-year ban for Pakistan batsman Latif|
|PSG ´destroyed the market´ with Neymar and Mbappe - Elber|
|Bulldogs chairman says sacked Hasler will not be compensated|
|Coutinho withdrawal inspired Leicester to victory, claims Shakespeare|
|Windies opener Brathwaite cleared to bowl|
|Valverde hopes Suarez benefits from Eibar rest|
|Wilshere returning to his best form - Wenger|
|Yankees, Cardinals keep pace in postseason race|