Parker finished 4-14 in the Big Eight and is 6-20 overall.
Parker held a 5-4 lead in the seventh inning but could not hold on. The Lancers scored a run in the seventh to send the game into extra innings.
In the ninth, the Lancers' A.J. Verhage ripped an RBI single to send the Vikings to defeat.
"We had the lead in the seventh inning," Parker coach Brian Martin said. "It's about the fifth or sixth time we've had the lead in the seventh and lost it. It's just one of those things, I guess.''
La Follette took a 3-0 lead in the second inning, but Parker scored twice in the fourth on Aric Bobzien's RBI ground out an error and three runs in the fourth on four hits, including three RBI singles from Jake Welch, Bobzien and Vernon Franklin to take a 5-3 lead.
Parker loaded the bases in the sixth, but failed to score and missed scoring opportunities in the seventh and eight innings. Parker left 10 men on.
Parker faces Monona Grove in a Division 1 regional game 5 p.m. Tuesday at Fireman's Park in Cottage Grove.
LA FOLLETTE 6, PARKER 5 (9)
Janesville Parker 000 230 000—5 9 0
Madison La Follette 030 001 001—6 10 4
Hyerdahl, Greene (4) and Welch; Ricci, Verhage (5) and Seelow.
Leading hitters—Bailey (JP) 2x4; Franklin (JP), 2x4; Pollard (MF) 2x5. 2B—Powers (JP), Seelow (ML).
SO—Hyerdahl 3, Greene 3, Ricci 4, Verhage 1. BB—Hyerdahl 3, Greene 4, Ricci 2, Verhage 2.