The Ethiopian looked to be heading for a time well below Ruth Chepngetich's world record in Sunday's Berlin Marathon when she suddenly started limping with two kilometers left to run.
There was talk of a suspected cramp, but it appears to have been a much more serious injury.
Gianni Demadonna, who represents the 29-year-old, told the AP news agency that Tigst Assefa ran to the finish line with a "totally destroyed right Achilles tendon."
She reportedly started to feel pain after completing 31 of the 42 kilometers. After passing 40, she could no longer run as usual.
She lost all strength as soon as she put her foot on the ground, says Demadonna.
She completely destroyed her Achilles tendon at the end.
Despite the injury, Assefa crossed the finish line in 2:11:04, the third-fastest marathon time by a woman of all time, 68 seconds off Chepngetich's world record.
Tigst Assefa underwent surgery on Monday in Murnau, southwest of Munich, Germany. According to Gianni Demadonna, she will need to walk with crutches for about six weeks, and will not be able to run again for about three months at the earliest.
