LONDON — Alex Meyer was in medal position halfway through this 6-mile aquatic mix-master — and then a rugby scrum broke out at the feeding station in the middle of The Serpentine on Friday afternoon. “I just got like totally pummeled,” said the 24-year-old Harvard grad, who’d prepped for his day on this 18th-century artificial lake by swimming solo laps at Walden Pond, Thoreau’s old bathtub.

Thus did his medal chances ebb away as Meyer dropped to 12th at the end of the fourth of six laps in the 10-kilometer open swim, 10 seconds off the pace. “I just kind of fell off the wagon a little bit,” said Meyer after he’d finished 10th. “I didn’t really have that extra little bit of magic to bring it home the last 5k.”