Littler had hammered world number one Humphries 6-1 on Sunday night, and shocked those in attendance when he said he'd be taking part in the World Youth Championship the following day.
Many felt the teenage superstar would cruise the event, but despite a 107.4 in his semi-final against Greaves, Littler was beaten by the two-time Women’s World Matchplay winner.
Greaves herself averaged 105, and though Littler came out firing with a pair of ten-dart finishes - and narrowly missed out on a nine-darter - she held firm before delivering an incredible 11-dart break of throw in the deciding set with a checkout of 80 to win the last-four thriller 6-5.
Greaves' win at the Robin Park Leisure Centre means she will be the first woman to play in a World Youth Championship final, where she will meet reigning champion Gian van Veen in Minehead next month.
"I had two 10-dart legs and missed double 12 for a nine darter and still couldn't win," Littler said on social media after his loss.
"Fair play to Beau Greaves. All the best in Minehead. Some talent."