Havertz, who has been away from the national team for more than a year after missing last year's internationals through injury, is part of the Germany squad to face Switzerland on Friday and Ghana three days later.
"I had a tough time. It was not easy for me," Havertz told a press conference ahead of Germany's friendly against hosts Switzerland.
"I had my first injury in February last year, three and a half months out then comeback and then a setback with the knee injury," he added.
Havertz returned to action for the Premier League leaders Arsenal in January after being sidelined since August last year, but the 26-year-old said he was ready for his club's busy match schedule in the league, the FA Cup and the Champions League.
"I now feel completely fit. I need the one or other game from the start to reach 100 percent but I feel fit to start lasting over 90 minutes," he said.
Havertz added that Arsenal's busy schedule came just at the right time.
"We have many matches in three competitions. I missed the first six months of the season and I will give it all for the club, and to be able to go with a lot of confidence to the World Cup. I want to be successful at this World Cup and lead the way," he said.
Germany have set their sights on winning the World Cup following shock group-stage exits in 2018 and 2022, determined to restore their reputation as a global football powerhouse. The tournament starts in June and is co-hosted by Mexico, the United States and Canada.
The Germans, who have not reached a final in any major international tournament since winning their fourth World Cup in 2014 in Brazil, have been drawn in Group E with Ecuador, Curacao and Ivory Coast for the June 11th to July 19th tournament.
They play their opening World Cup match in Houston, Texas, on June 14th against Curacao. Germany will then travel to Toronto on June 20th to take on the Ivory Coast before their last group game against Ecuador in New York on June 25th.