Goals from Josef Martinez, Franco Escobar and Hector Villalba sealed a massive advantage for Atlanta, who picked up a 3-0 win at Mercedes-Benz Stadium in Sunday's playoff showdown. The win is Atlanta's first over the Red Bulls, with the Supporters' Shield champions picking up three wins and a draw in the team's four previous matchups.
Following a fairly tepid start, Atlanta finally broke through in the 32nd minute on a finish from Martinez.
The Venezuelan, who broke the MLS record for goals during the regular season, scored his 35th of the season to give the hosts the vital opening goal.
On a ball over the top from Jeff Larentowicz, Red Bull defender Tim Parker misjudged his jump and it fell to Martinez. The Atlanta forward took a touch before firing from close range, giving Atlanta the 1-0 lead.
That lead appeared to evaporate in the second half as Bradley Wright-Phillips fired home on a set piece in the 53rd minute. However, following video review, the goal was waved off as Alex Muyl was deemed to be offside and impacting the play in front of Brad Guzan.
With the lead restored, Atlanta continued to push forward in an effort to double the advantage. The Five Stripes finally did in the 71st minute on Escobar's finish. Julian Gressell played the initial ball, which skipped past Martinez and several Red Bull defenders on the way to Escobar on the back post.
Once there, the defender rocketed a first-time shot, leaving Luis Robles helpless as Atlanta scored the all-important second goal.
The third all but ended the Red Bulls' chances as the league's best defense was beaten once again in stoppage time. Villalba was left in too much space 25 yards out, and the substitute winger fired a shot into the bottom corner to seal a lopsided win that will likely take Atlanta through to hosting MLS Cup.
With that, Atlanta will take that 3-0 lead to Red Bull Arena for the second leg, which is scheduled for Thursday.
Neither the Portland Timbers nor Sporting KC will have any such lead in their Western Conference clash as the two sides played to a 0-0 draw at Providence Park in their opening leg.
A physical and cagey affair, both sides were strong defensively throughout, with the Timbers looking the more likely to score but never finding a way through.
Sporting KC goalkeeper Tim Melia had a number of big saves, including a Bill Tuiloma header in the last 10 minutes that prompted a diving stop.
The Timbers' best look did slip past Melia, as the 2015 MLS Cup champions appeared to have taken the lead in the 70th minute on a David Guzman finish.
However, that goal was called back for offside, helping pave the way to Sunday's scoreless draw and Thursday's winner-take-all clash at Children's Mercy Park.
