Getting the top values by group

Here's a sample data frame:

d <- data.frame(
  x   = runif(90),
  grp = gl(3, 30)
) 

I want the subset of d containing the rows with the top 5 values of x for each value of grp.

Using base-R, my approach would be something like:

ordered <- d[order(d$x, decreasing = TRUE), ]    
splits <- split(ordered, ordered$grp)
heads <- lapply(splits, head)
do.call(rbind, heads)
##              x grp
## 1.19 0.8879631   1
## 1.4  0.8844818   1
## 1.12 0.8596197   1
## 1.26 0.8481809   1
## 1.18 0.8461516   1
## 1.29 0.8317092   1
## 2.31 0.9751049   2
## 2.34 0.9269764   2
## 2.57 0.8964114   2
## 2.58 0.8896466   2
## 2.45 0.8888834   2
## 2.35 0.8706823   2
## 3.74 0.9884852   3
## 3.73 0.9837653   3
## 3.83 0.9375398   3
## 3.64 0.9229036   3
## 3.69 0.8021373   3
## 3.86 0.7418946   3

Using dplyr, I expected this to work:

d %>%
  arrange_(~ desc(x)) %>%
  group_by_(~ grp) %>%
  head(n = 5)

but it only returns the overall top 5 rows.

Swapping head for top_n returns the whole of d.

d %>%
  arrange_(~ desc(x)) %>%
  group_by_(~ grp) %>%
  top_n(n = 5)

How do I get the correct subset?