Greenbank won bronze in the 200m backstroke at the Tokyo Games three years ago but breached the 15-metre limit for swimming underwater at a lap-change and was disqualified.
The distraught Greenbank crouched by the pool-side after it was confirmed.
“I’m speechless. I can't really say... I have no words," he told Eurosport.
Bidding for her second gold medal after winning the 400 individual medley title, Canadian teenager McIntosh was sixth quickest in the morning heats, with China's Olympic champion Zhang Yufei topping the timesheets in 2:06.55.
"This morning I was just trying to make semis and conserve as much energy as possible heading into tomorrow's final," McIntosh told reporters.
"Yesterday, since I had a day off competing, I was able to reach out to a bunch of my friends back home.
"Now I'm all in focusing on the 200 butterfly."
Switerland's Roman Mityukov was fastest into the 200 backstroke semi-finals, while Rio Olympic champion Ryan Murphy also safely advanced.
China's Xu Jiayu, who won silver in the 100 backstroke in Paris, did not start in his 200 heat.
Fresh from winning the women's 100 breaststroke gold on day three of the meet, South Africa's defending champion Tatjana Smith was fastest in the 200 semi-finals, clocking 2:21.57.
Already her nation's most successful Olympic swimmer with two golds and a silver, Smith can equal Chad le Clos as South Africa's most decorated Olympian with another medal in the 200.
"I think that 100 (gold) really took off a lot of pressure," she said. "So now this time I can really just have fun with my swimming, enjoy every single race," said Smith.
The evening session on Wednesday will feature five gold medals, highlighted by Katie Ledecky's defence of her 1,500 freestyle title.
American great Ledecky can match former US swimmer Jenny Thompson's women's record of eight Olympic golds with victory.
Romania teenager David Popovici, the 200 freestyle champion, will go for a second gold in the 100 as will Australia's Mollie O'Callaghan in the women's 100 event.
Home hero Leon Marchand, the 400 IM gold medallist, will try to become the first swimmer to win both the men's 200 breaststroke and 200 butterfly golds.
