A 57th-minute strike from Los Angeles-born El Salvador international Nathan Ordaz handed LAFC a precious advantage heading into next Wednesday's return leg in Florida.

"I think we saw a complete performance," said LAFC coach Steve Cherundolo, whose only complaint was that his team had not managed to win by a more convincing margin.

"If we're honest with ourselves we need to be a little sharper in front of goal. If we get the same types of chances next week in Miami we need to be a little more ruthless."

Miami coach Javier Mascherano had few complaints about his team's defeat.

"Obviously we didn't have the best night tonight but it's only the first leg," Mascherano said.

"We have another game next week. We can do what we need to reach the semi-finals."

Unbeaten in nine games across all competitions since the start of the season, Messi and Miami arrived in Los Angeles as the form team of Major League Soccer.

But after a cagey first half that saw neither side manage to get on top, Los Angeles raised their intensity and began causing problems for the visitors.

LAFC fullback Ryan Hollingshead squandered a golden chance in the 54th minute, blasting well wide after bursting into the penalty area on the overlap.

They made the breakthrough three minutes later with the 21-year-old homegrown striker Ordaz spinning away from former Barcelona and Spain international Sergio Busquets and thumping a low shot into the bottom corner past Miami keeper Oscar Ustari.

Argentine superstar Messi, making his first start since returning as a substitute last weekend following a two-week injury layoff, struggled to gain a foothold in the contest.

Instead it was Los Angeles who looked likeliest to score in front of a raucous home crowd of 22,207 fans at the BMO Stadium that included NBA superstar Stephen Curry, Miami team owner David Beckham and Argentina national team manager Lionel Scaloni.

US international Aaron Long headed just over from a free-kick in the 63rd minute, and moments later only a desperate goal-line block from Miami defender Maxi Falcon denied LAFC midfielder Timothy Tillman.

Falcon had one of Miami's few chances in the second half, glancing a header towards the bottom corner that LAFC's former France international goalkeeper Hugo Lloris gathered comfortably.

In a sign of mounting Miami frustration, Busquets was booked for a wild challenge on Cengiz Under.

A disappointing night for Messi was summed up by his failure to make the most of a free-kick in a promising position deep into stoppage time, with the Argentine sending his shot whistling well over the bar as LAFC held on for a deserved victory.