Michael Chang beat Lendl with an underhand serve when cramps hit

On 5 June 1989, 17-year-old Michael Chang, unseeded, was two sets down to Ivan Lendl, the world No. 1 and three-time champion, in the fourth round of the French Open. Late in the fourth set his legs began cramping so severely he could barely run for a ball, in an era before on-court medical timeouts existed for cramping and with no realistic way to substitute out and recover: his entire game, built on speed and retrieving, had lost its central weapon mid-match against one of the best players alive.

The conventional response to cramping against a fitter rival is to keep competing on the same terms and hope adrenaline covers the gap, or to accept that fitness alone will decide the outcome. Chang instead started hitting exaggerated high lobs to shorten every rally and buy recovery seconds between points, clawing the match to a decisive fifth set — but serving at 4-3, 15-30, with his legs no longer able to generate a normal service motion, he faced a service game he had no physical way to win on ordinary tennis terms.

Switched to a legal underhand serve, changing the contest from physical to tactical, exploiting an opponent unprepared for it.

The underhand serve required no leg power, so Chang could execute it despite cramping.

Lendl had never faced such a serve, so he had no rehearsed response and returned it awkwardly.

The shot's novelty forced Lendl to solve an unfamiliar problem, breaking his rhythm.

Changing the contest's terms let Chang compete on psychology and tactics, not lost fitness.

When your usual approach fails due to resource depletion, a legal but unconventional move that disrupts the opponent's expectations can buy time and shift momentum.

Chang won the game and, from that turning point, the match 4-6, 4-6, 6-3, 6-3, 6-3 after four hours and thirty-nine minutes, one of the biggest upsets in French Open history. Six days later he beat Stefan Edberg in the final 6-1, 3-6, 4-6, 6-4, 6-2 to become, at 17 years and 3 months old, the youngest man ever to win a Grand Slam singles title, a record that stood for decades.