Semenya, who is challenging a new hyperandrogenism rule by the International Association of Athletics Federations (IAAF), took nearly a second off her personal best as she claimed her 25th consecutive final win in the event. Burundi's Francine Niyonsaba was second in 1:55.86.
"I did not expect that," Semenya, who ran without pacemakers, told reporters. "I was thinking 1:54.99 could be possible but this was great."
The IAAF rule, against which Semenya has appealed to the Court of Arbitration for Sport (CAS), would effectively give her a choice of taking medication to restrict her testosterone level or move to longer-distance events. It is scheduled to become effective in November unless overturned by CAS.
"I am just a human and if you want to be an inspiration to the world and to the youth, you cannot focus on negative things," Semenya said.
Only Czech world record holder Jarmila Kratochvilova (1:53.28), Ukrainian Nadezhda Olizarenko (1:53.43) and Kenyan Pamela Jelimo (1:54.01) have run faster than the South African.
Samba became just the second runner to break 47 seconds in the 400 metres hurdles. The other was American world record holder Kevin Young, whose 46.78 won the 1992 Olympics.
"I made a small mistake at the start, lost my balance on the first hurdle so I did not expect to run so fast," said the 22-year-old, whose previous best was 47.41.
Russian world champion Mariya Lasitskene also continued her dominance, winning her 45th consecutive high-jump competition with a sparkling season's best of 2.04 metres.
All told, seven yearly bests were set or equalled at the meeting.
American world champion Sam Kendricks soared 5.96 metres to win a high-quality pole vault over young Swede Mondo Duplantis (5.90) and French world record holder Renaud Lavillenie (5.84) and American Ronnie Baker gained a share of the year's 100-metres top time with a personal best 9.88 seconds.
Kenyan world silver medallist Timothy Cheruiyot became the first 1,500-metres runner to dip under 3:30 this year as he easily won in 3:29.71, and countrywoman Beatrice Chepkoech led a Kenyan sweep in the women's 3,000-metres steeplechase with a personal best 8:59.36.
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Neville Dalton)
