"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 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.
"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.