Jenson Button expects Red Bull driver Sebastian Vettel to be McLaren's biggest rival for pole position in Singapore.
Vettel set the pace in both free practice sessions on Friday, with Hamilton second fastest in the first and Button his nearest challenger in the second.
But while the 2009 world champion did not rule out other contenders, he expects Red Bull to continue its strong street track form.
Asked by AUTOSPORT if Vettel was going to be his biggest rival this weekend, Button replied: "Apart from my team-mate, yes.
"Sebastian has been consistently quick here. Red Bull seem to be very good on street circuits and they have been all year."
The 32-year-old believes that Ferrari driver Fernando Alonso will also be a factor.
He was particular impressed with the Spaniard's long-run pace, meaning that the Ferrari could be a threat on Sunday in particular.
"I don't think he [Vettel] is going to be the only person that's competitive. Fernando will be up there as well," said Button.
"His long runs look reasonably competitive. I think they'll be good."
Button added that the unpredictable nature of Formula 1 this year means that a team that struggled on Friday could still get into contention.
"You never realy know in F1 these days. Maybe Sauber will spring an attack from nowhere.
"The Lotuses I see were not happy yesterday but that can easily turned around today."
Hamilton said on Friday that he was happy with the handling of his McLaren, but Button admitted that he still has work to do on the balance.
- Sebastian Vettel
- Red Bull