Flextable group by column

I am producing reports through Rmarkdown that need to be able to be rendered to Word. I am hoping to be able to reproduce this table format:

I have tried using flextable to do this without any real headway. I've read through the flextable site and have found a way to use groups for merging rows, but not to group columns.

The as_grouped_data() function can create the groups beforehand, but can I use these groups for column headings?

Small example of my data:

df = structure(list(Athlete = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 
1L, 1L, 1L, 2L, 2L, 2L), .Label = c("Athlete 1", "Athlete 2"), class = "factor"), 
    Time = structure(c(1L, 2L, 3L, 1L, 2L, 3L, 1L, 2L, 3L, 1L, 
    2L, 3L), .Label = c("1", "2", "3"), class = "factor"), Measure = structure(c(1L, 
    1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("Distance", 
    "Speed"), class = "factor"), Value = c(4.02, 11.5, 19.82, 
    3.03, 9.67, 17.9, 6.5, 8.08, 8.47, 5.3, 7.64, 8.67)), row.names = c(NA, 
-12L), class = "data.frame")

# Create table
myft = flextable(df) 
# Merge athlete column
myft = merge_v(myft, j=c("Time", "Athlete"))
myft

I am stuck here and unsure how to swivel these groups from row headings to column headings.

To create the format you desire for your table, you probably want the data "wide." This will give you 2 columns for your 2 athletes. This example uses pivot_wider from tidyr.

After arranging by Time order, you use your merge_v from flextable. Added fix_border_issues to fix bottom table border after merge.

library(flextable)
library(tidyverse)

myft <- df %>%
  pivot_wider(id_cols = c(Time, Measure), names_from = Athlete, values_from = Value) %>%
  arrange(Time) %>%
  flextable()

# Merge Time column vertically
myft = merge_v(myft, j="Time")
myft = fix_border_issues(myft, part = "all")
myft

Output