Sainz, who is leaving the Italian team at the end of the season to make way for seven-time world champion Lewis Hamilton, is only four points behind Leclerc despite missing the Saudi Arabian Grand Prix due to appendicitis.
The Spaniard has finished on the podium in all three of the races he has started, and won in Australia on his return from surgery.
"I think it's as simple as he's doing a better job," Leclerc said when asked at the Chinese Grand Prix on Thursday whether Sainz was adapting better to the car than him.
"In Bahrain (the opening race) it’s difficult to compare because on my side I was facing issues and I think it was a very strong weekend apart from that, on my side. However, in the last two races he’s just been stronger.
"It's up to me now to work, especially in the qualifying pace which is normally a strength. I have been struggling to put the lap together. It's a very fine line to get it right or completely wrong on the out-lap and putting the tyres in the right window and for now I have been struggling more than what Carlos has done.
"He's driving at a very high level, which I think is great for the team."
However, Leclerc is confident that he'll be able to improve things imminently.
"So I’m not worried, but obviously now I need to show that on track starting from tomorrow in qualifying," he added.
Ferrari are second in the constructors' standings, 21 points behind Red Bull, whose triple world champion Max Verstappen has won three of the four races and started them all on pole position.
Sainz finished third in Bahrain, with Leclerc fourth, and third in Japan where Leclerc was also fourth. In Australia, Ferrari finished one-two.
Leclerc started ahead of Sainz in Bahrain but the Spaniard got the better of his teammate in qualifying for Australia and Japan.
Leclerc said China, returning to the calendar for the first time since 2019, should be better for Ferrari than the previous race in Japan although he still expected Red Bull to have the upper hand.
