Williams is expecting increasingly better performance from Kazuki Nakajima in 2009.
The Japanese rookie, 23, made his debut at the end of 2007, and although involved in some incidents, and having been outscored by his teammate Nico Rosberg, has impressed many observers.
"Kazuki has had his learning period," Williams' chief engineer Rod Nelson is quoted as saying by Formule 1 Race Report.
He insisted that Nakajima's development has been steady in 2008.
"When you compare the Nakajima from the beginning of the season to the end of the season, you see right away that he has grown tremendously," Nelson said.
He also said he is "very pleased" Williams has retained both drivers for 2009.
"It is just convenient," he said. "Good or bad, you should always improve compared with the previous season."