Even champions need to seek out redemption sometimes.

Stanley Biwott was in the final stretches of the Rio Olympic marathon when he couldn’t go any farther. The special water that had been made for him — a sugary mix — had been confused with that of fellow Kenyan Wesley Korir. “It was tasting [like] citrus,” he said Friday, as he prepared to defend his New York City Marathon title.

For finely-tuned running machines, both of whom were among the leaders, the swap was enough to short-circuit the whole system; the stomach pain, he said, was intolerable. And so Biwott did what he’d never done before in a competitive race: He dropped out. Korir did, too.

“It was a mistake,” Biwott said of the swap. “I was ready for the Olympics and also I’m ready for New York . . . As the defending champion, I’m proud to be back. I don’t have pressure because I’ve trained good for the race.”

Biwott, 30, is the favorite in the men’s race, which kicks off Sunday morning. He took first last year, in 2:10:34, and had the best performance of his career this year at the London Marathon, which he won in a personal record of 2:03:51.

The women’s defending champion, Kenyan Mary Keitany, won in in 2:24.25 last year. A win this weekend would be her third straight, a feat that hasn’t been accomplished since Norway’s Grete Waitz won five straight, ending in 1986. (Waitz won nine New York City marathons, the most of any runner)

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“Winning on a course like this is not easy,” Keitany said. “I mean to try my best. My training has been going well, so I think I’m going to try my best, and if it’s to win the third one, I think that would be good to me.”

Like Biwott, Keitany has her own hiccups to overcome. Sapped from a recent illness, she fell and finished ninth in the London Marathon in April and was left off the Kenyan Olympic team. Though extremely soft-spoken — it is often hard to hear her, even when she is close — she is a fierce athlete.

“I was very disappointed when I lost in London,” she said. “I wanted to go to the Olympics again, so I said ‘OK, let me just go to New York and defend my title.’ ”

The marathon presents a challenge of a different kind for Sally Kipyego, who, like Keitany, is one of only 33 professional female runners to make up the expected 50,000-plus participants on Sunday. A 2012 Olympic silver medalist in the 10,000 meters, she ran her first New York City marathon last year and didn’t finish.

“Sign me up for the next one; that’s the first thing I told my coach,” she said. “It didn’t discourage me. It just made me respect the distance a little bit more and it made me realize this isn’t a 10K . . . I’m a competitive athlete. I never race to just participate. No. I’m too competitive for that. I race to compete and win and be a factor.”