It was only Game 1, but it may have been the defining game of the series.

For 30 minutes, the San Antonio Spurs seemed in total control. Racing out to a 20-point lead over the Lakers in Game 1 of the Western Conference finals, it seemed the defending champs were ready to mark their turf and steal a road win right out of the gate.

And then, just like that, the game turned completely upside down.

San Antonio scored only 20 points in the final 17 minutes and 54 seconds of the game, blew the entire lead, and lost to the Lakers 89-85 on Wednesday. Kobe Bryant alone had more points (23) than the entire Spurs team in that stretch, after he’d managed only four in the first two and a half quarters. Meanwhile, the Spurs scored only nine times in their final 36 possessions, continuing a season-long pattern of second-half offensive meltdowns.

It was either an epic comeback or an epic collapse, depending whose side you were on, but the result is the same: The Lakers hold a 1-0 series lead heading into Friday’s Game 2, and perhaps an even larger edge when you consider the opportunity the Spurs squandered.

Since it only gets three home games in the best-of-seven format, San Antonio has to win at least once in Los Angeles in order to claim the series. Wednesday might have been its best shot. L.A. has been nearly unbeatable at home since acquiring Pau Gasol (they haven’t exactly been slouches on the road, either), but with four days off they seemed unusually rusty coming into this one.

Bryant in particular was shockingly passive for a player of his caliber, seemingly deferring to lesser teammates for no reason and rarely attacking off the dribble. That showed in the box score — L.A. has been a dominant offensive team since the Gasol trade, but mustered only 45 points through two and a half quarters, a miserable 72-point pace.

Once Kobe woke up, it was a different story. In the final 18 minutes, L.A. scored nearly as many points (44) as in the first 30 minutes, which Bryant scoring more than half and assisting on several others.

Analysts undoubtedly will focus on the psychological impact of the Spurs’ collapse heading into Game 2, but that angle may be overplayed. Locals need only hearken back to one of the biggest comeback in playoff history, when the Nets punted a 26-point lead against Boston in Game 3 of the 2002 conference finals. New Jersey came back to win the final three games and take the series — no psychological damage there.

Additionally, it’s hard to imagine a team with the Spurs’ mental toughness and proven resiliency to cave just because things didn’t go their way in the opener.

So instead of psychology, let’s look at performance. While Bryant and the Spurs’ Tim Duncan (30 points, 18 rebounds) were brilliant in Game 1, several other important players on both sides were much less so. Which ones bounce back in tonight’s sequel at Staples Center may hold the key to who prevails. For instance:

San Antonio’s Manu Ginobili was perhaps the most prominent one to struggle. He shot only 3-for-13 from the field and had four turnovers. Most of the turnovers were bad ones, too, with him forcing passes into crowds for no apparent season. Ginobili has been bothered by a sore ankle for several weeks, although it seemed to improve during the New Orleans series, and he had a fingernail torn off at some point during Game 1 (ouch), so his physical condition could be a contributing factor,

Another factor, however, is how L.A.’s Sasha Vujacic played him. For years coaches have instructed players to take away Ginobili’s left hand and force him right, but that’s proven easier said than done. Vujacic took this technique to extremes in Game 1, with great success. He stood nearly parallel to Ginobili, practically on top of his left hand, and gave him no choice by to start every move by going right. Vujacic played 32 minutes off the pine and will be counted on heavily again, as other than Bryant he’s the team’s most effective wing defender. Ginobili will have to punish Vujacic for leaving the gate open for his right-hand drives by making quick, effective moves in that direction, and of course he’ll have to ease up on the passes in traffic.

The Lakers’ Derek Fisher also had a rough night, after playing outstanding basketball in the second round against Utah. His outside shooting touch has been particularly sharp of late, so it was surprising to see him hoist one brick after another en route to a 1-for-9 shooting night. Fisher seemed particularly trigger-happy off the dribble, forcing several jumpers early in the shot clock when he might have probed for a better look. He’ll need to be more patient tonight and go back to feasting off the scraps Kobe throws his way.

San Antonio’s Michael Finley struggled too, missing all five of his 3-point tries and getting beaten by Lamar Odom for a key basket late in the fourth quarter.

His woes mirrored those of the entire Spurs bench. San Antonio only got 11 points out of the unit in 71 minutes, with four rebounds and four assists. As a group they shot 5-for-17 and didn’t attempt a free throw. While the Spurs rely on heavily on Duncan, Ginobili, and Tony Parker to do the heavy lifting on offense, they need far more production from Finley and the rest of the subs.

Finally, the Lakers’ Odom was another who had a rough Game 1: He shot only 3-for-12 and missed two of his four foul shots. Plus, the trick that worked so well in the Utah series — attacking their big men off the dribble — failed in Game 1 because Duncan was able to thwart his forays to the rim. Odom needs to find different ways to get his points against the more versatile Duncan, possibly by shooting more jumpers and getting away from Duncan’s comfort zone near the rim.

I suspect Ginobili, Fisher, Odom, and Finley will all bounce back tonight — their track records are too strong for them not to. But if so, that puts San Antonio right back to where it started — playing on the road against a team with more talent and more weapons.

That’s why the comeback/collapse on Wednesday stings so much for them. Stealing the opener in L.A. was their best hope of getting the road win they’d need to take the series. Instead they’re down a game and down to three bullets for that road win. One suspects those numbers will be two and two after the Lakers win again — albeit in less dramatic fashion — tonight.

jhollinger@nysun.com