The German told Sky Sports News: "For today, I think it was a good day, a stronger day for us. It's very difficult where we are in terms of pace but it's really easy to say that we're not with Mercedes.
"We had a strong day today, no issues with both of our cars nothing went wrong, a lot of things were learned. Now it's up to us to see some improvements tomorrow.
"They've proven again today that they're quite quick so it will be a long way for us to catch up but in the long term it's important that we make bigger steps than them."
Qualifying for Sunday's race takes place tomorrow.