"I am not trying to be Superman," said Gay, who clocked a wind-assisted 20.14 seconds in his bid to run a sprint double at the August 10-18 championships in Moscow.
Although he managed only a couple hours' sleep after his speedy 100 metres win over Olympic bronze medallist Justin Gatlin, Gay told reporters: "After I warmed up this morning I decided to go ahead (and run) because I am healthy. That's the key."
His time was the third fastest behind Isiah Young (20.09) and Curtis Mitchell (20.11). All were wind-assisted.
Gatlin, who is nursing a strained hamstring, chose not to run the 200.
"9.8 on a leg and a half is good enough for me," Gatlin said of his 100 metres time.
The 200m semi-finals and finals are on Sunday, the concluding day of the meeting.