Czech snowboard great Ledecka fails in bid for third straight Olympic gold
The world champion topped qualifying in the PGS, a head-to-head race through gates on two parallel courses marked red and blue.
The 30-year-old then cruised through her last-16 contest but came unstuck in the quarter-finals against Austria's Sabine Payer.
Ledecka made a mistake near the top of the course and Payer held onto her lead, winning by 0.06 seconds.
No snowboarder has ever won three straight Olympic titles.
Ledecka already has a unique place in Olympic history as the only woman to secure gold in two different sports at the same Games after winning the parallel giant slalom and alpine skiing's super-G at the 2018 Pyeongchang Games.
She is likely to compete in the Super-G later at the Milan-Cortina Olympics.
Ledecka is lobbying to keep PGS on the Olympic programme, with the sport, introduced at Salt Lake City in 2002, facing an uncertain future.
She said in an Instagram post that she had chosen to prioritise snowboarding at the 2026 Games "because it's an amazing sport, which for sure belongs to the Olympics".