TUKWILA — Half of the 14 MLS teams that played U.S. Open Cup third-round games Tuesday lost to lower-division opponents.

For Sounders FC, it was a useful warning before the three-time defending champion of the tournament easily avoided its own upset scare.

Forward Sammy Ochoa scored twice in the second half and Seattle also got goals from Andy Rose, Osvaldo Alonso and Alex Caskey to cruise past the Atlanta Silverbacks of the second-tier NASL, 5-1, Wednesday in front of 3,703 spectators at Starfire Stadium. It was a team-record scoring performance for an Open Cup game.

The Sounders made a point of talking before the game about the many upsets throughout the competition.

"It's a game that you can never take too lightly, because the moment you take it lightly, that's when you're going to get caught," said coach Sigi Schmid.

Seattle will benefit from another upset in the fourth round.

Cal FC, a fifth-division amateur team, was an unlikely 1-0 winner Wednesday on the road against the Portland Timbers and will come to Starfire for a 7 p.m. game next Tuesday. Ticket information will be made available Thursday.

From the opening whistle, the Sounders were on the attack as Ochoa hammered a shot on goal at the 15-second mark. The attempt was saved, but indicative of the awaiting onslaught — though it took until just before halftime for the goals to arrive.

A half worth of pressure finally resulted in reward in the 44th minute, when Rose was set up for a shot from the top of the penalty box by defender Jeff Parke. He smashed a half-volley by Atlanta goalkeeper Joe Nasco, who was frozen by the exceptional strike.

"The past four games we haven't been able to score first," said Rose, referring to Seattle's four-game MLS winless streak, "so that was the main thing, get the first goal."

Added Ochoa: "We knew when we had one the rest were going to come."

They did and quickly. Alonso doubled the lead in the 47th minute when he collected a pass on the right side of the penalty box from Cordell Cato and drove a low shot past Nasco.

Atlanta pulled a goal back in the 53rd minute through experienced forward Reinaldo Navia, but the Sounders would answer immediately on a great individual run and finish by Caskey in the 54th minute.

"Seeing some of the results from yesterday — (MLS) teams went up and then let up a little bit and let the (lower-division) teams come back — it was lucky that we were able to score a couple right away and kind of demoralize them a little bit," said Caskey, a native of Dunwoody, Ga.

Ochoa capped the scoring with goals in the 62nd and 66th minutes, both with his left foot. The Sounders finished with a 24-8 shot advantage.

"We were a little bit unlucky not to get more," said Schmid, noting nine saves made by Nasco.

The Silverbacks, who sold hosting rights for this game to the Sounders last week, were always going to be the underdogs. Atlanta is tied for last place in the NASL with a 0-4-5 record and now is 0-3 against MLS teams in the Open Cup.

Seattle didn't make things easy, starting a first-choice lineup defensively and many regulars in the attack. The Sounders, playing in their alternative Super Cyan uniforms, improved to 14-0-1 in Open Cup-related games — and in thrilling fashion.

"This," Parke said, "is our tournament."

NOTES

• The Sounders play host to Vancouver in an MLS Reserve League game at 6 p.m. Friday and resume MLS play June 16 in Montreal.

• Atlanta features two local products in defender Raphael Cox (Tacoma) and midfielder Ciaran O'Brien (Federal Way). Cox played for Stadium High School, Highline Community College and the University of Washington, and also has trained with the Sounders. O'Brien is a Decatur High alum and played on loan for the USL Sounders in 2008.

Joshua Mayers: 206-464-3184 or jmayers@seattletimes.com. On Twitter @joshuamayers.