Wayde van Niekerk's hopes of a golden double remain on course as the newly-crowned world 400m champion scraped into the final of the 200m on Wednesday.
The South African will be joined by Botswana's Isaac Makwala, the fastest sprinter over 200m this season.
The Botswanan was forced to run an additional solo qualifying time earlier in the evening as the IAAF had barred him from competing in the initial 200m qualifiers (and also the 400m final) because he had been diagnosed with the highly contagious norovirus and was put under a 48-hour quarantine.
There was drama for Van Niekerk, however, as he struggled and could only finish third in his heat, eventually qualifying for the final as one of the two fastest third-place finishers thanks to his 20.28sec time, beating out France's two-time Olympic bronze medallist Christophe Lemaitre by just 0.02sec.
"I knew it would be a tough challenge," said Van Niekerk.
"To see my name in the final is a real pleasure. I've got time to recover now and give it my all in the final."
In pouring rain at a packed-out London Stadium, Azeri-born Turk Ramil Guliyev won the third semi-final in 20.17 seconds, with American Ameer Webb finishing second ahead of Van Niekerk.
The South African will have to quickly regroup for Thursday's final if he is to have a real chance of completing a first world 200/400m double since Michael Johnson in Gothenburg in 1995, a feat the American also repeated a year later in the 1996 Olympics in Atlanta.
American Isiah Young won the first semi-final in 20.12sec, with Makwala second at 0.02sec. Third-placed Briton Nethaneel Mitchell-Blake also qualified for the final with his time of 20.19.
"I'm still running with my heart broken," said Makwala.
"I wish the IAAF had given me the decision to run the 400m first. I was ready to run. I don't know who made the decision. 400m is my reason for training.
"I'm running with anger, 400m is my race.
"But thanks to the IAAF for letting me run today."
The second semi was won by Trinidad and Tobago's Jereem Richards in 20.14sec ahead of Japan's Abdul Hakim Sani Brown, with Yohan Blake missing the cut, meaning there will be no Jamaican to carry on Usain Bolt's form, having won the last four world gold medals in the event.
"I thought I could go through in first or second but I'm alright," said Blake.
"We just have to keep rebuilding. We have to come back for the relays and stay focused."