Commonwealth Games champion Nijel Amos of Botswana also safely moved through to Sunday's semi-finals with victory in a time of 1 minute 47.23 seconds in the first heat at the Bird's Nest Stadium.
"It was a good run, I wanted to stay out of trouble. I take each round as it comes. It is a bit hot here but we can cope," he said.
Olympic champion Rudisha led from start to finish as he clocked a time of 1:48.31 in the sixth and final heat which was also surprisingly the slowest.
"I have qualified and now I am looking forward to the final," he said of his showdown with Amos and Aman.
"I know it is going to be hard. I know my power is back. My problem before was my finishing speed."
The diminutive Aman offered 1 min 47.87 in a physical and competitive second heat featuring lots of jostling for position.
Earlier, Aman's compatriot Genzebe Dibaba had no such troubles negotiating the heats of the women's 1,500m as she clocked a best time of 4:02.59 across the three heats.
The 24-year-old world record holder took the lead at the bell and raced clear with Faith Kipyegon of Kenya to record the fastest women's 1500m time outside a final in world championships history.
"This is a special championships in a special city because my sister won two golds here at this unique stadium," she said of 2008 double Olympic champion and sibling Tirunesh.
"So my main goal is to get two gold medals too."
The heptathlon begun with the 100 meters with young Dutchwoman Nadine Visser clocking a personal best in winning with a time of 12.81 seconds ahead of Briton Jessica Ennis-Hill (12.91) and Canadian Brianne Theisen Eaton.
Olympic champion Ennis-Hill, though, led after the morning session following a seasons best 1.86 metre effort in the high jump.
Compatriot Katarina Johnson-Thompson was the only woman to clear 1.89 in the high jump as she moved into second overall ahead of Visser.
In the hammer, Pawel Fajdek and Briton Nick Miller were the only men to achieve the 77 metre automatic qualifying mark for Sunday's final, while the first round of the men's 100 meters gave an opportunity to some of the lesser names to enjoy the grand stage.
Bhutan's Tashi Dendup recording the slowest time across the three heats of 12.15 seconds but also a national record for the Himalayan nation.
(Writing by Patrick Johnston; editing by Amlan Chakraborty)
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