In-form Irish striker Robbie Keane struck twice for Galaxy as they beat Seattle Sounders 3-0 in the Western Conference, while Dynamo enjoyed a 3-1 win in the home leg of their Eastern Conference championship against D.C United.
The pair met in last year's final, which Galaxy won 1-0, and it now requires a big turnaround from D.C or the Sounders to stop a repeat.
Dominic Kinnear's Houston had to come back from a one-goal deficit but they did so in emphatic fashion.
Rookie Nick DeLeon, whose goal against the New York Red Bulls on Thursday earned D.C their place in the conference final, struck again with a 27th-minute opener.
DeLeon fed Lionard Pajoy down the left who cut inside and hit the right post but DeLeon responded quickly and slotted in the rebound.
Two-times MLS champions Houston went close to an equaliser eight minutes before the break when Macoumba Kandji's diagonal shot from the left was pushed wide by Joe Willis.
That was a sign of things to come in the second half.
Houston pulled level when Honduran Oscar Boniek-Garcia burst in from the left and slipped the ball across the box to Andre Hainault, who slotted home confidently.
The Dynamo grabbed the lead through another close-range effort from Will Bruin in the 68th and then after some poor defending from a corner, Kofi Sarkodie's right footed shot from inside the area made it 3-1.
The second leg will be played on November 18 at RFK Stadium in Washington.
Seattle showed little ambition against Los Angeles, lining up in a defensive formation clearly hoping to limit the damage in the away leg.
The tactic appeared to be working in a dull first half until Keane struck on the stroke of half-time, heading home from close range after good work from Landon Donovan.
Keane then turned creator with a fine ball out wide to the overlapping Sean Franklin, whose low cross was driven home by Mike Magee to make it 2-0 in the 64th minute.
The Donovan-Keane combination bore fruit again three minutes later when the American bore down the left. His deflected shot was turned goalwards by Swede Christian Wilhelmsson, with Keane adding the final touch.
Donovan had a goal-bound effort cleared off the line by Jeff Parke as Galaxy threatened a fourth but the Sounders will still need a remarkable turnaround in Seattle next week if they are to reach their first MLS Cup final.
