Andy Murray says keeping his world number one ranking will be “difficult” because of the depth of competition at the top level of tennis.
Briton Murray, 29, holds a healthy lead over Novak Djokovic as he begins his Barcelona Open campaign on Wednesday.
However, his year has been hampered by illness, injury and poor form.
“I’m happy to be fit and healthy again and hopefully I can start to play some good tennis again soon,” the Scot told BBC Sport.
“It’s always hard to maintain your ranking at the top of the game.
“You have some of the best players of all time playing just now and playing great tennis.
“A lot of the young ones are starting to play better and better – so it’s going to be tough.”
Murray plays in Barcelona on Wednesday, when he faces the winner of the match between Dustin Brown of Germany and Australian Bernard Tomic.
He lost to Albert Ramos-Vinloas in the third round of the Monte Carlo Masters last week and, as the clay-court season builds towards the French Open next month, he is looking to play more competitive matches.
“I want to try to get matches in,” said Murray. “I haven’t played in the last five or six weeks. That’s the reason for coming here.
“The best way to adapt to new surfaces [is] playing naturally against the best players in the world.
“It doesn’t matter how many times you practise, it’s getting the matches in that counts. My job is to try to win matches this week and hopefully I can do that.”
Murray initially appeared to struggle with his serve after his return from an elbow injury, but the Scot believes that is improving.
“I was happy with how it felt last week,” he said. “I didn’t have loads of time to practise the serve beforehand, but it felt good and I served better as the matches went on. I’m sure here it will be better again.”
Murray is seeded to meet world number five Rafael Nadal, who will also play his first match on Wednesday, in Sunday’s Barcelona Open final.
The Spaniard, who beat Ramos-Vinloas in the Monte Carlo final, has won the French Open nine times.
“His results on clay everywhere are very special,” Murray said. “He’s dominated all over the major clay-court events throughout his career.
“To win 10 times in one place, I don’t know if it’s ever been done, but I’d be surprised if it had because it is a very difficult thing to do.”
Jamie Murray’s frustrating year continued after he and partner Bruno Soares were knocked out after a match tie-break, 7-6, 2-6 (10-3), in the first round in Barcelona by Fabrice Martin and Edouard Roger-Vasselin.