Andre Hainault celebrates after tying the game in the 51st minute. (Bob Levey/Getty Images)

For two months, D.C. United has been defying long odds and broad skepticism to rekindle memories of championships past. Even when things went bad, through injuries, suspensions and scheduling mayhem, United discovered ways to extend an exhilarating season.

But with a 3-1 loss to the Houston Dynamo in the first leg of MLS’s Eastern Conference finals on Sunday — another chaotic playoff affair blemished by controversy and ailments — United will have its mettle tested like never before this fall.

In a two-game series decided by total goals, D.C. will need to win by two next Sunday at RFK Stadium to force overtime and, if necessary, penalty kicks.

“If you don’t have any faith in this team yet, you’re blind,” defender Robbie Russell said. “We may not be the prettiest, we may not do the things the way everybody wants sometimes, but we fight, and we are going to fight next weekend.”

The fight will surely be there, but some of United’s key players might not. Leading scorer Chris Pontius departed Sunday’s match in the 12th minute with a groin injury. Midfielder Marcelo Saragosa was gone before halftime with a hamstring problem. And Brandon McDonald, United’s premier defender, left early in the second half with a sore calf.

The rest of the squad barely made it to the final whistle, its energy sapped by an intense series against New York, travel to Texas and Houston’s warmth and humidity. “I’m exhausted,” said midfielder Nick DeLeon, United’s goal scorer. “I couldn’t breathe out there.”

All three injured players will be evaluated throughout the week. Their potential absences further depletes a roster that has been missing 2011 league MVP Dwayne De Rosario since early September with a knee injury and will again be without suspended right back Andy Najar.

De Rosario might suit up for Game 2 and goalkeeper Bill Hamid will return from a red card suspension.

“In no way are we out of this thing,” Coach Ben Olsen said. “The beauty of it is we can score two goals, especially knowing we need to score two goals. Next week should be a lot of fun.”

Facing a Houston team that hadn’t lost at home this season, United would’ve been ecstatic about earning a draw in the series opener. D.C. had never won in nine previous visits to Houston, but for one half, the club was exceeding hopes, thanks to DeLeon’s goal in the 27th minute.

The match turned late in the first half — not on a goal, but a non-call by referee Ricardo Salazar. United’s Raphael Augusto, a Brazilian midfielder making his MLS debut in place of Saragosa, was dragged down by Andre Hainault just beyond the penalty area. The act probably warranted a red card. Instead, Salazar’s whistle remained silent.

United fumed on the sideline and, at halftime, the coaches had words for Salazar, who, from a distance, directed them to the tunnel.

“Everybody in the stadium, everybody on our bench, everybody on their bench, everybody at home saw that it’s a red card,” Olsen said. Commenting on Augusto’s first game and the disputed sequence, Olsen added: “He makes the play that should change the entire course of the series.”

Hainault was a prominent figure throughout the afternoon. On DeLeon’s one-timer from 15 yards, the ball had caromed off the Canadian defender’s foot and past goalkeeper Tally Hall.

Six minutes after intermission, Hainault scored the equalizer. Off a quick restart, Boniek Garcia rushed unmarked into the left side of the penalty area and crossed to Hainault at the six-yard box for a simple finish.

United almost retook the lead in the 66th minute, but Lionard Pajoy’s one-timer was cleared off the goal line by defender Bobby Boswell. Two minutes later, Houston went ahead when Giles Barnes beat Dejan Jakovic on the end line and delivered a low cross. Goalkeeper Joe Willis slapped the ball, but Will Bruin converted the rebound — his fourth goal of this postseason.

United wasn’t going to win this game — and was going to have trouble tying it — but paramount to the total-goals series was preventing a greater deficit.

Willis made a reflex save on Kofi Sarkodie, but in the 81st minute, United’s failure to clear a corner kick allowed Sarkodie to beat Willis to the far corner from eight yards.

“We gave up some soft goals, some concentration lapses, mistakes that we haven’t had in a while, so that’s concerning,” said Olsen, whose squad had conceded just six goals during a 6-0-3 streak heading into Sunday.

Said Willis: “We’re down two goals in the series — it’s halftime. We’re going back home. It hurts, but it’s something we can overcome.”

GALAXY 3, SOUNDERS 0: In Carson, Calif., Robbie Keane scored twice, and Mike Magee also found the net as Los Angeles beat Seattle to take a commanding lead in the first leg of the Western Conference finals.

Keane had an assist and Sean Franklin set up two goals by the defending champion Galaxy, who will head to Seattle next Sunday seeking its MLS Cup appearance in four years.

The Sounders need to win by four goals next weekend to advance.

— Associated Press

Dynamo at United

Sunday Nov. 18, 4 p.m., RFK Stadium