Petke rose up completely unmarked at the edge of the six-yard box to head home a well-placed corner from Terry Cooke into the far side of the net for the eventual game-winner.
For the moment, the victory pulled Colorado all the way from last place in the Western Conference into a tie for third with Chivas USA, though Chivas plays at Toronto FC on Saturday. FC Dallas is now in fifth place, a point behind Colorado and Chivas.
Colorado interim head coach Gary Smith made several changes to his starting 11. In the back, Jordan Harvey got his first start since a 2-1 loss at Columbus on June 28 and Kosuke Kimura was in the first team for the first time since June 14.
In the midfield, winger Cooke returned after missing last week's 2-0 loss at Real Salt Lake with a left hamstring injury and up top, Tom McManus returned for his first start since July 12 at San Jose.
FCD head coach Schellas Hyndman, who was coaching his 10th game for the Hoops, also made some alterations to his starting 11. Defensively, Pablo Ricchetti moved from holding midfield to center back as captain Duilio Davino continues to be sidelined with a toe injury that has forced him to miss four matches.
With Adrian Serioux away with the Canadian national team for a World Cup qualifier, Drew Moor moved from the middle to right back. In the midfield. Michael Dello-Russo got just his second career start on the right side, in place of Andre Rocha. The Brazilian midfielder was not in the 18-man gameday squad in what was called an internal decision by the coaching staff.
FCD rookie Bruno Guarda got his second straight start at the attacking midfield spot while Marcelo Saragosa, who filled in for Ricchetti last week while he was on suspension, returned as the holding midfielder. Another surprise came on the left, where Anthony Wallace got the starting nod, his first action since May 24.
A quiet first half saw neither team greatly threaten their opponent's goal. In the 28th minute, Cooke struck a perfect corner that found McManus. The Colorado striker's shot might have gone high but Sala still got a hand on it in an acrobatic dive.
The home side came close in the 33rd minute when Kenny Cooper whistled a shot from about 25 yards out that brought a save out of Chivas USA goalkeeper Preston Burpo.
In the 35th minute, Jeff Cunningham found himself staring at an empty net when a bid by Burpo to strip him of the ball near the arc resulted in the Rapids 'keeper falling to the ground. Cunningham recovered the ball wide of the goal, and with a trio of Rapids players streaking back toward the goalmouth, the FCD forward sent an off-balance effort that bounced off the corner of the right crossbar and bounded harmlessly over the endline.
After neither coach made any changes to start the second half, Colorado nearly drew first blood in the 47th minute. After the ball bounced around the FCD area for what seemed like an eternity, Casey got possession about eight yards from goal but Aaron Pitchkolan was there at the right post to deny Casey's bid.
Cooper was heard from again in the 49th when he struck a nice ball from about 25 yards out from the right flank. The shot was aimed just inside the near post, but Burpo was there once again to deny the Dallas native.
Both coaches went to their respective benches not soon after. Hyndman summoned rookie Eric Avila for Wallace in the 52nd while Smith brought on John DiRaimondo for Pablo Mastroeni at the same time for the visitors.
In the 60th minute, Cooke swung in a from the right. Kenny Cooper appeared to have the responsibility for marking Petke, and when he got tangled with another player, Petke was left all alone to calmly head the ball back inside the right-hand post for his first goal since Sept. 8, 2007. For Cooke, it was his team-leading eighth assist of 2008.
After Petke's goal, Pitchkolan made a bid to draw the Hoops level in the 61st minute, but his effort was right at Burpo and the Colorado 'keeper was on point for the easy save.
Hyndman called on Dax McCarty in the 65th for Dello-Russo, and from the moment he came on the field he sparked the FCD attack. From a corner in the 80th minute, he found Moor inside the Rapids box but the FCD defender's shot sailed high. He later found Moor with a skipping ball through the box, but Moor put his diving header from close range just outside the left-hand post.
In the 83rd, McCarty again provided a spark when he spotted substitute Dominic Oduro open inside the Rapids area, but Oduro's attempted header went high over the goal.
In the 90th, Moor was whistled for a hard tackle on Colorado's Colin Clark. That gives the FCD defender five yellow cards, which means he will miss the Hoops' next match at D.C. United on Sept. 13.
Even with some five minutes of stoppage time to end the match, the best chance in the end went to the visitors when defender Facundo Erpen found himself wide open just outside the FCD box, but his shot curled wide of the left post.
FCD doesn't return to league action until Sept. 13 when they take on D.C. United at RFK Stadium, though they will play Mexican side Club America in a friendly at Pizza Hut Park on Sunday. Colorado next plays on Sept. 14, when they face the Chicago Fire at home.
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