Sha'Carri Richardson and Shericka Jackson on course for 200m showdown
On another roasting morning Richardson, seeking to become the first American winner since Allyson Felix in 2009, looked sharp before easing up to win her heat in 22.16, the fastest time of the six races.
Fourth-placed 100m finisher Marie-Josee Ta Lou of the Ivory Coast went through from the same heat, while defending champion Jackson was in cruise control winning her heat in 22.50.
Gabby Thomas, quickest in the world this year with 21.60 to win the U.S. trials in July - making her the fourth-fastest woman in history - looked superb in winning her heat in 22.26.
"I felt very comfortable. It's my first time on the track and I managed to get out strong and come through," Thomas said.
"I think I can run my world lead time again when I need to but I don't think that will be enough to win gold. I think all the girls in the final will be under 21.60, so it's going to be fast. The track is really fast, it really is.
"I feel like I'm in the best shape I've ever been in so I'm ready to roll. Being part of women's sprinting now can have its ups and downs, you can run extremely fast and still lose a race but it's really exciting and having that pressure brings out the best in all of us."
Briton Dina Asher-Smith, the 2019 champion and bronze medallist last year, also went through with 22.46 and echoed Thomas's views on the sport at the moment.
"As long as you're on the start line for the next round you have a chance," she said.
"The talent level in this event now is off the scale, so it's amazing to be even considered among the best in the world. I do think in 40 or 50 years people will look back and say, 'That era was amazing'."
Asher-Smith also said the athletes were having to work hard to deal with the heat.
"I'm using an ice vest, ice towel, shade - those are the basic tricks," she said.
"Obviously I've got to warm up and make sure everything is working as efficiently as possible but even this morning I had to save something because this saps your energy and with the semi-finals tomorrow it's going to be fast."
Jamaican Shelly-Ann Fraser-Pryce, silver medallist last year and who took bronze in the 100m on Monday, opted not to run in 200 but plans to return to the track for the sprint relay.
Poland's Krystsina Tsimanouskaya, who defected from Belarus after her high-profile fallout with her home country at the Tokyo Olympics, finished fifth in her heat in 22.88 but advanced as one of the fastest losers, having gone out at the heat stage in the 100m.
The semi-finals are on Thursday, with the final on Friday.
