
Temperatures touched 27 degrees Celsius (80F) as Diamond, a world silver medallist with Britain in the women’s 4x400m relay, ran her preliminary heat on Australia’s Gold Coast.
The 26-year-old qualified in third in her race, but she was in visible discomfort and lay prone next to the track afterwards with a pack of ice on her to cool her off.
“I got heatstroke in the warm-up and was throwing up,” Diamond said afterwards.
“Even in the shade it is 25 degrees. I was just trying to stay as cool as possible. I need to get a (wet) towel for around my neck.”
Diamond called her performance “OK”.
“I was watching the girl alongside me and just tried to hang onto her,” she added.
The 400m semi-finals are on Tuesday.