The Danish 10th seed was leading 7-6 (7-1) 4-3 when Peng appeared to suffer severe cramp and withdrew from her maiden grand slam semi-final.
It was a sad ending for the Chinese who stunned three seeded players en route to her Arthur Ashe Stadium showdown with Wozniacki.
And Peng had proven more than a match for the former world number one in a contest that lasted over two hours, and in which the underdog drew first blood.
She broke first to lead 4-3 but her momentum was immediately halted as Wozniacki, finalist in 2009, hit back in the very next game.
Peng showed remarkable resolve to bounce back from that and put herself in a position to serve for the set at 6-5, but Wozniacki - seeking a first grand slam crown - produced the goods when it mattered to take it to a tie-break.
And there the 24-year-old demonstrated the kind of skill and composure under pressure that once took her to the top of the rankings, storming to a 7-1 victory in the breaker to seize the initiative.
Again it was Peng who struck the first blow in the next set, racing into a two-game lead only to surrender the advantage immediately as Wozniacki reeled off four straight games to move to within touching distance of the final.
It was clear before then that Peng was starting to struggle and she required a medical timeout when sitting on a break point.
When she resumed play, Peng evidently was not in the best shape and, despite being able to face five more points - even getting another break point when Wozniacki double-faulted - she had to call it quits in front of a sympathetic and appreciative crowd.