Jenson Button believes Lewis Hamilton's ability and pace could surprise Nico Rosberg when the pair team up at Mercedes next year.
Button became the first of Hamilton's team-mates to beat him over a season in 2011, finishing runner-up as Hamilton ended the year in fifth.
This year Hamilton has regained the ascendancy, finishing directly above Button in the championship and ending the season with a 15-5 qualifying advantage even after penalties at Shanghai and Barcelona.
While Rosberg and Hamilton have been team-mates before back in 2000, Button warns that Hamilton's sheer speed will be the hardest thing for the German to acclimatise to.
"I think he'll probably get a bit of a surprise," Button said when asked what Rosberg could expect.
"I know he's been his team-mate in the past, but I think he'll be surprised at how Lewis can get performance out of a bad car.
"I didn't say that [Mercedes will be bad] - he [Hamilton] can also get speed out of a good car, and I think that will be the biggest thing."
Button and Hamilton were involved in a Twitter spat earlier this year following Hamilton's tweeting of confidential McLaren data over the Belgian Grand Prix.
Button said Rosberg knew what to expect when it came to working with the 2008 world champion, adding: "I think Nico knows Lewis better than I do.
"They've worked together before; I think he knows what to expect."
Asked whether he will rue Hamilton's departure from McLaren, Button said: "I'm not disappointed because there's always change.
"I'd be disappointed if Paddy Lowe moved from the team because that's the guy who's going to help me achieve in the future.
"Whereas with Lewis, drivers come and go and people change, and that's pretty normal in the sport.
"It's been a good three years and it's amazing how quick those three years have gone by. It's been a good partnership.
"We've had quite a bit of fun in terms of racing over those three years."