Burk reached the first checkpoint walking his horse in -- that shoulder of his had become a real problem.
"It wasn't the most comfortable ride, but I guess I will get used to it," he said. "More pain pills."
But then there was more bad news. At each station, a veterinarian performs a thorough check-up on each rider's horse and Burk's horse's heart rate was too high to allow him to continue, which meant he had to stay put until the horse's heart rate returns to normal, costing him time.
Meanwhile, by the end of Day One, Horn had made it to the third station -- 24 miles ahead of anyone else. By Day Two, Prior-Palmer made it to the second horse station.
By Day Two, the Gilbert-Fretelliere duo too had made significant progress, but when they reached the 8:30 p.m. cutoff for riding, they found themselves nowhere near a horse station. Their choices were to sleep rough or hope for nomadic hospitality from a Mongolian family.
"They say this is something you should do when you do the race--go with a family that doesn't know you're coming," Gilbert said. "I'm not accustomed to just walking into somebody's house and expecting them to feed me and give me a place to sleep."
Then came reports that Horn fell off and lost her horse, which can be a huge penalty, but miraculously she found it in the Mongolian wilderness and got back on. In addition, she was feeling ill.
"At like two o'clock I got the worst stomach flu in the history of man," Horn said. "I was puking and having diarrhea and it was terrible… And I couldn't get off my horse."
Her misfortune allowed Prior-Palmer to close the gap on her lead, putting them in a practical dead heat at a critical point in the race.
Meanwhile, Burk was still clinging on. Despite the searing pain in his shoulder, he made it over 200 miles and to nine horse stations. But as the gauntlet slogged into Day Five, he was dealt a derby fiasco.
"So I'm walking again, my horse's cinch was slipping a bit so I got off to tighten it up and then I tried to get back on, it bucked me off and took off back to the beginning of the herd," he said.
He had no choice but to turn back.
"I think I might be dead," Burk said. "I was getting back on and really, really twisted my shoulder. It kind of was up, and back. And I mean, it's been a lot more painful than I've been letting on."
At his breaking point, Burk was the sixth rider to quit. The others who were still in the race were wishing they weren't. A stomach bug that had hit Horn was wreaking havoc on the other riders. After Gilbert came down with diarrhea, her partner, Fretelliere said she was disappointed to continue alone.
"I wanted to finish together," she said. "It's really sad, but let's see the positive. She can still catch up to me."
But it wasn't easy. One after the other, the Mongol Derby took riders down until only 18 remained. Of those, the battle for first place was down to just two, Horn and Prior-Palmer, and it was neck to neck between the two, both riding at a punishing pace, until something shocking happened.
Tune into "Nightline" TONIGHT at 12:35 a.m. ET to find out what happens.