Ferrari have won the last three races - the first time since 2008 that Scuderia have won three consecutive races - and Mercedes haven't won since before the summer break, with the Italian team's car proving too quick for the constructors' championship leaders.
Hamilton, who finished in P4 in Singapore, "doesn't know" when Mercedes will use their upgrades to their car and believes the next few races will be difficult for the Silver Arrows.
"I'm sure we do have upgrades coming at some stage," the five-time world champion said to BBC Sport. "I don't know how much is coming, but these next few races are going to be tough."
Hamilton leads the drivers' standings by 65 points, with his team-mate Valtteri Bottas his nearest competitor, but Max Verstappen and Charles Leclerc are slowly creeping up on the Brit; 96 points adrift.
"It hasn't really changed much for me; it's just as hard," Hamilton said. "You won't realise how hard we work in the background. Even the races we've won have not come as easily as you probably perceived it.
"There is so much work in the background, to get the starts, to get the strategy right, to get the car in the (set-up) window. Of course, it's got tighter and tighter as the season has gone on. But at the same time we just have to be doing a better job. We were doing such a great job at the beginning of the year."