Replace missing values (NA) in one data set with values from another where columns match

I have a data frame (datadf) with 3 columns, 'x', 'y, and z. Several 'x' values are missing (NA). 'y' and 'z' are non measured variables.

x    y z
153  a 1
163  b 1
NA   d 1
123  a 2 
145  e 2
NA   c 2 
NA   b 1
199  a 2

I have another data frame (imputeddf) with the same three columns:

 x  y z
123 a 1
145 a 2
124 b 1
168 b 2
123 c 1
176 c 2
184 d 1
101 d 2

I wish to replace NA in 'x' in 'datadf' with values from 'imputeddf' where 'y' and 'z' matches between the two data sets (each combo of 'y' and 'z' has its own value of 'x' to fill in).

The desired result:

x    y z
153  a 1
163  b 1
184  d 1
123  a 2 
145  e 2
176  c 2 
124  b 1
199  a 2

I am trying things like:

finaldf <- datadf
finaldf$x <- if(datadf[!is.na(datadf$x)]){ddply(datadf, x=imputeddf$x[datadf$y == imputeddf$y & datadf$z == imputeddf$z])}else{datadf$x}

but it's not working.

What is the best way for me to fill in the NA in the using my imputed value df?