The German set the fastest time in the first practice session in Sochi, but his Mercedes teammate has had stronger pace since, helping him secure pole position earlier.
The 29-year-old told reporters: "Today Lewis was quicker, all weekend actually. Tomorrow I have to try to get close, but I didn't manage to get closer than two tenths.
"Also I had Bottas coming quick behind me, so I need to keep an eye on that. But the front row is good and there is hardly any tyre degradation because the track is so smooth on the tyres and that makes it a unique challenge to set up the car to get the grip."
Formula 1 is taking place in Russia for the first time.