Mercedes could not challenge Ferrari's pace and took the fourth and fifth positions respectively.
Ferrari have won the last three races to add pressure
Ahead of the Singapore GP, Leclerc won the Belgian and Italian GP respectively to deny Hamilton and his Mercedes team member Valtteri Bottas.
Therefore, the Briton is feeling the pressure at the moment and feels Ferrari will provide a stern challenge for the rest of 2019.
There are six more races left in 2019 that includes the Russian GP this weekend.
These next races are going to be tough: Hamilton
"These next races are going to be tough," said Hamilton.
"If anyone feels relaxed they need talking to because we should all be feeling the pain in our team. I think I am a realist so I see the situation we are in - and Ferrari have come up with some sort of upgrade that's given them a massive boost."
Hamilton might not have it easy in 2019
Hamilton leads the points tally in 2019 (296). However, Leclerc (200), Vettel (194) and Bottas (231) can all catch up with six races to go. Therefore, what looked like an easy run in for a sixth world championship, has suddenly got more interesting.
It's going to be very hard to beat them: Hamilton
"Clearly, their car works everywhere now so it's going to be very hard to beat them, particularly as they are so fast on the straights. At the moment, they are delivering better than us so we have to get back together," added Hamilton.