Haas duo Romain Grosjean and Kevin Magnussen have praised the "surprising" progress that Force India has made with its recent upgrades.
A raft of changes made to Force India's car at the Singapore Grand Prix has helped its fight for points and, after Esteban Ocon ended up best of the rest in Friday practice in Japan, Grosjean said he was not expecting his rival to be so strong here.
"Force India was a bit surprising," he said. "They have been fast recently after a very difficult beginning of the year, and they have raised their game quite a long way since they changed name."
Magnussen has no doubt that Force India has made a leap in pace to join the fight at the front of the midfield battle.
"I think it is pretty clear isn't it?" he said. "They've made a very big step and done an extremely good job to turn it around, so we will see.
"I don't think we are far behind but it looks like we are a little bit behind, so we will try to make up for it perhaps tomorrow or the coming races."
Asked to comment on Sergio Perez's suggestion ahead of the Japanese GP that he believed Force India now had the best midfield car, Grosjean said: "Okay! Well, it's great if we can beat them then!
"I don't know. They have certainly been quick and they are very, very, very fast in a straight line. So some circuits you can explain, like Spa and Monza and Russia. Singapore is a bit of a different one.
"But they have done a good job, honestly, getting that car back on track as they did, and here we will see what it is like."
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- Formula 1