The Spaniard kept the title race technically open by finishing fourth as Vettel stormed to a fifth straight victory at Suzuka.
"I don't think [the title fight is still on]," Alonso said.
"Even if Sebastian retired from all four [remaining] races, I would have to win all of them.
"We keep doing the best possible, but it's a matter of time [until Vettel wins the title].
"We weren't as quick as the Red Bulls or Grosjean so fourth was the maximum.
"We recovered some of the pace we didn't have earlier in the weekend, but the top three were too strong for us even without traffic."
HOW VETTEL CAN WIN THE CHAMPIONSHIP IN INDIA
Vettel currently leads Alonso by 90 points and will become champion if he places fifth or better in India.
Alonso's gap to Vettel after India must be 74 points or less for the championship to continue to Abu Dhabi.