Reigning F1 world champion Sebastian Vettel said that Red Bull had had a good day of practice for the Chinese Grand Prix, but did not believe that he would be able to challenge current pacesetters Mercedes when it mattered.

The German finished fifth fastest in the second session of the day, with RBR team-mate Daniel Ricciardo one place better off, but remained seven-tenths off the pace of Mercedes' Lewis Hamilton. Although the gap was smaller than at other points this season, Vettel insisted that there was still a lot of ground to make up.

"Maybe [Mercedes] didn't run full beans today," he offered, "It's difficult to say, but I think they're still the favourites and still quite far away.

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"I was behind the Mercedes on the long run and it looked like they could do what they wanted. Of course, it depends a lot on which tyres you have on the car, and which fuel setting. Nevertheless, I think, for us, it was a good day, but we're not yet on the level of the top guys."

Buoyed by a strong Bahrain test after round three of the season, Red Bull continues to suffer a power deficit courtesy of its Renault engine, and Vettel insisted that there had been no major step forward on that front.

"We have a couple [of new] bits and we are trying all the time to make a step forward , not only on the performance side, but also on the tuning of all the bits and bobs," he acknowledged, "[In terms of power unit improvements], it's mostly stuff in the background. It's not as if we've gained power. Obviously, we'd love to, but that's not really possible. Tuning the bits we have feeds the feeling a lot and that's where we're still looking for [improvements]."

Shanghai International Circuit is renowned as being 'front tyre limited', and Vettel admitted that Red Bull was in the same boat as everyone else when it came to trying to control wear.

"We were pretty average today [in terms of tyre wear], probably the same as everyone else," he commented, "I haven't seen what they did but they probably suffered the same issues as we suffered. I think it will get better as the weekend goes on, but it depends what weather we have tomorrow and Sunday. If it's dry both days, then the circuit will ramp up but, if it's wet, it will reset every time.

"However, it was a good day in terms of testing, [even though] the track was very green. There's still a lot for us to do - and for me especially, as I'm not yet where I want to be, but we made some good progress today."