Sebastian Vettel said Ferrari has "some catching up to do" after struggling to match Formula 1 title rival Lewis Hamilton for pace through second practice for the Russian Grand Prix on Friday in Sochi, finishing more than half a second off the pace.

After topping FP1 earlier in the day for Ferrari, Vettel slipped back to fifth place in the second practice session at the Sochi Autodrom as Hamilton headed up a one-two finish for Mercedes, with the Red Bull duo of Max Verstappen and Daniel Ricciardo taking P3 and P4 respectively.

Vettel called his day "tricky" when speaking to reporters after the session, conceding that he felt he had struggled both over one lap and in the long runs through FP2.

"Obviously we looked quite far away from the rest of the field, so [it's] not ideal. I think struggling a bit for one lap pace, but also in the long runs," Vettel said.

"I think we went through our tyres a bit harder and quicker than the rest, so we need to have a look. But also just looking at ourselves, it wasn't a good day in terms of feel for the car. I think there’s more that we need to go to."

Vettel was heard venting his frustration about on-track traffic that hindered his runs through FP2, but the German said it did not make a great deal of difference to Ferrari's day overall.

"That’s Friday for you, sometimes you have a better Friday, sometimes not. I think for us it wasn’t a particularly clean run in the end, but still good enough," Vettel said.

"I think as I said, we struggled with the tyres with or without traffic. Certainly the traffic doesn’t help.

"But I think we have some catching up to do."