Red Bull's team principal Christian Horner has conceded that he has been impressed with Lewis Hamilton's form during pre-season testing.
The 2008 champion, who joined Mercedes at the end of the last campaign, was one of the fastest drivers during recent sessions in Spain.
"Mercedes have recorded some head turning times with the programme they were operating to," Horner is quoted as saying by BBC Sport. "Their car looks quick and with Lewis joining the team they will naturally take a step forward."
The new campaign will get underway in Australia on Sunday, March 17.
