Fernando Alonso will find it "very tough" to hold off Sebastian Vettel in their battle for the Formula 1 drivers' title, according to Jenson Button.
Red Bull's Vettel closed Alonso's lead to just four points by winning the Japanese GP while the Ferrari retired.
"It's going to be very tough to hold on to Sebastian," McLaren's Button said.
"It's gone his lead. It's also tough for him because Red Bull are so fast. Ferrari are pretty quick but not as quick as Red Bull."
Alonso spun out at the first corner after his left-rear tyre was punctured by the front wing of Kimi Raikkonen's Lotus.
It is the second time in four races that Alonso has been forced into retirement through no fault of his own - he was taken out by Raikkonen's team-mate Romain Grosjean at the start of the Belgian Grand Prix last month.
The Spaniard said: "It was a shame. It is always sad when you cannot do the first corner but we need to concentrate and think about [the Korean Grand Prix] next week.
"We need to keep working well and not making mistakes. Nothing we can do. Thanks to this consistency we are leading the championship. The others make mistakes, we need to avoid this."
Vettel's victory in Japan meant he drew level with Alonso and McLaren's Lewis Hamilton as the only men to win three races this year.
It was also his second victory in succession following a win in Singapore two weeks ago.
But although the Red Bull was by far the fastest car in Japan, the German dismissed suggestions he was now the favourite for the title.
"Obviously it was an important step, but there is still a long way to go," Vettel said.
"You don't wish these things on people. It's a long season and you don't know what's going to happen.
"It could be us in the next race. We have seen this year there is a lot of up and down and things can change quickly so we have to keep our head down and go step by step and not think too far ahead.
"It could be completely different next weekend, so we cannot rely on that [pace]. Obviously it was fantastic this weekend. We were surprised ourselves how quick we were."
Red Bull team boss Christian Horner said: "Fernando is a very shrewd and formidable opponent, and over a season luck tends to balance itself out.
"Sebastian has had two DNFs as a result of reliability, Fernando has had been unlucky with Grosjean and the puncture he picked up today.
"Luck tends to balance itself out and it will be down to what the two of them do on the track and the relative performances of the cars over the last five races."
Ferrari have had some problems in recent races with planned upgrades not working as intended, and team boss Stefano Domenicali urged his team to concentrate on the job ahead.
He said: "The most important thing is to stay really rational and not fall into the worst enemy of the team, what the others are trying to put on us, which is pressure.
"It is a very important point of the season, and we have the opportunity to react straight away next Sunday in Korea. Heads up, we need to be very cool."
Alonso is 37 points ahead of third-placed Raikkonen with 125 points still available in the remaining five grands prix.
McLaren's Lewis Hamilton is fourth, 42 points behind Alonso and 18 ahead of Red Bull's Mark Webber. Button is sixth, 63 points ahead.