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."
|Gattuso surprised by AC Milan´s growth since taking over|
|Lambert defends ´best goalkeeper in Britain´ Butland after own goal error|
|Stokes expected to face Black Caps|
|Mkhitaryan, Ozil potential excites Mertesacker|
|A-League Review: Perth Glory in dramatic win, Melbourne Victory end losing streak|
|Leicester City 1 Stoke City 1: Butland howler costs Potters|
|Matic insists Man Utd can still do ´big things´|
|Fisher´s birdie streak secures share of Qatar Masters lead|
|Chinese Super League transfer window makes Simeone ´uncomfortable´|
|Ingolstadt score bizarre tap-in after Duisburg goalkeeper goes AWOL|
|Allegri cautious over Dybala fitness despite Higuain injury|
|Napoli´s Zielinski better than De Bruyne, claims Boniek|
|Seven-try Crusaders beat Chiefs, Waratahs and Brumbies win|
|Mourinho predicts ´amazing´ period for Manchester United|
|Mears expects ´good England win´ at Murrayfield|
|Barcelona midfielder Rafinha hoping to seal permanent Inter move|
|EFL Cup final: Where do Aguero and Aubameyang rank among Europe´s elite?|
|Klaasen and Mulder get Proteas Test call to face Australia|
|Settled Salah demands Champions League qualification for Liverpool|
|England not polished article - Shaw|
|Dembele staying focused after ´genius´ praise from Pochettino|
|Spain survive scare, USA cruise in World Cup qualifying|
|Shapovalov moves into Delray Beach Open semis|
|Davis, Jokic star with huge performances|
|Alli doesn´t dive, says Hodgson|
|Timberwolves star Butler suffers knee injury|
|Penguins acquire Brassard in three-team trade|
|Verdasco ends Thiem´s Rio Open title defence, Monfils exits|
|Salah has world´s best left foot? Don´t forget Messi – Klopp|
|Pochettino: Vertonghen one of Europe´s best defenders|
|He hasn´t raised the bar – Wenger plays down Guardiola success|
|Woods stays solid as List, Lovemark lead|
|Cake and victory for birthday boy Pouille|
|Klopp ´really proud´ of Firmino after Holgate case dropped|
|Munster upset Warriors as Leinster crush Southern Kings|
|Bouchard ´vindicated´ after USTA settlement|
|Warriors fall to impressive Wolves, Saints stay perfect|
|Conte: Chelsea´s winning mentality can´t be questioned|
|Benching Willian was a fantastic decision, boasts Chelsea boss Conte|
|Juventus critics don´t know Tottenham, says Pjanic|
|Hull City 1 Sheffield United 0: Dicko blunts Blades´ promotion chase after fan protest|
|Bastareaud stars as France get off the mark against Italy|
|Jaguars reward Marrone & Coughlin with two-year extensions|
|Lukaku tipped for ´big things´ by United team-mate Matic|
|Kasatkina saves three match points to stun Muguruza|
|Emery hopes Classique can inspire Real Madrid comeback|
|I thought he was an actor! - Valverde dodges Arthur questions|
|Valverde hints at resting Messi, Suarez for Girona clash|
|Napoli sign free agent Milic|
|Barclay labels England ´massive favourites´ for Calcutta Cup|
|Johnson tells England: Don´t give Scotland ´scent of victory´|
|Zidane offers condolences after Basque police officer´s death|
|FA charge Guardiola over yellow ribbon protest|
|City have to lift titles - Guardiola focused on Wembley glory against Arsenal|
|Spalletti urges Inter to ´believe in our strength´ and end wretched run|
|Best unconcerned with Sexton fitness|
|Christensen a future Chelsea captain, says Conte|
|Fonte leaves West Ham for Dalian Yifang|
|Guardia Civil search Guardiola family´s plane|
|Unwell Neymar facing race to make Le Classique|
|Playing Chelsea means less and less every year, says Mourinho|
|Conte hopes Manchester United bench ´fantastic´ Pogba|
|Dembele ´a genius of football´ like Maradona and Ronaldinho, claims Pochettino|
|Otaegui surges into slender lead in Qatar|
|Villarreal suspend Semedo after attempted murder allegation|
|Jesus could return but Guardiola has doubts over Sterling|
|He took 10 seconds to be ready – Mourinho salutes professional Pogba|
|Alderweireld a doubt as Pochettino stays silent on transfer rumours|
|Conte dismisses Mourinho tension|
|Nothing is decided - Klopp urges against Liverpool complacency|