Rosberg (left) has tested for Williams before

Williams chief operations engineer Sam Michael said the 18-year-olds had done well after completing 1,000km at Jerez.

"Nelson and Nico did a good job and worked well with the team, as well as showing themselves to be technically competent," said Michael.

Piquet's father Nelson and Rosberg's father Keke are both former champions.

Finn Rosberg set his fastest lap of one minute 19.468 seconds on Wednesday.

I was quite satisfied with how I got on Nelson Angelo Piquet

Brazilian Piquet's best of 1:20.453 was on Tuesday, and his time was three seconds slower than Williams driver Ralf Schumacher.

Piquet lapped the faster of the pair on Thursday.

Rosberg, whose father won the title in 1982, had tested previously for Williams after winning last year's German Formula BMW title.

"This week was great fun. It was a really unbelievable experience," he said.

"Of course there was a lot of pressure but I decided not to worry about that and just do the best I could."

Piquet, whose father won one of his three titles with Williams in 1987, said: "It went really well, particularly today despite it being wet for some of the time.

"I was quite satisfied with how I got on."