Lefty

verb (simile: to choke)

The term 'Lefty' is associated when you lead or dominate a game/tournament for the majority of the duration but you choke at the finish and lose. Doing a 'Lefty' is usually preceded by arrogant boasting which results in the following 'choke' being even more embarrasing.

'I thought he'd win the championship, but he did a Lefty'