Valtteri Bottas believes his Mercedes Formula 1 car was not "100%" in the Singapore Grand Prix's second practice after it was rebuilt following a crash in the opening session.
Bottas hit the outside barrier at the tight Turn 19 right-hander after he lost control of his Mercedes under braking, and it brought his first practice session to an early conclusion.
The crash meant Bottas ended the session four tenths behind Mercedes team-mate Lewis Hamilton, but that gap grew to more than a second in Friday's final practice.
Bottas admitted he felt "friction in the steering system" during second practice and believed the car did not feel right following its rebuild.
"It was a tricky one," Bottas said of his Friday. "Had a bit of an off moment in FP1, which ended my running a bit early, but everything was feeling OK until then.
"I just lost the back end in one of the corners, and obviously here with no runoffs, you pay the price.
"FP2, [I'm] not sure really what was going on. I was lacking pace, every single run, every tyre compound, so I'm not quite sure if everything was right in the car, [we] need to have a look.
"Just felt like I couldn't go any quicker, but I was one second off the pace.
"The only thing I could really feel - I just felt quite a bit of friction in the steering system. So, I struggled to feel the car properly.
"I doubt everything was 100%, but obviously [we] need to have a look [at] what I can do better as well."
Hamilton said driving his Mercedes at Singapore was "a shock to the system" due to the heat, humidity and the length of the lap - but he still topped second practice after finishing the opening session in third.
"I was really happy," Hamilton said. "I felt much better in the car today than I have for a while. I hope that [feeling] stays over the weekend.
"But we've definitely got some areas where we can improve, which is always good.
"And nothing's ever perfect, so I've got to go and study areas we can improve on the car, but also areas I can improve, like braking, turning points, etcetera."
Hamilton pinpointed the threat of Red Bull at Singapore after Max Verstappen finished just two tenths behind the Mercedes driver in second practice.
"I was quickest in that session, but they [Red Bull] were very, very close behind us," said Hamilton.
"Whether or not he had a perfect lap or whether we were more turned up than them, I'll find out now [in the debrief]."