Bang. Swish. Swish. Swish. Free throw. Layup. Layup. Swish.

Ballgame.

In the blink of an eye, America turned a competitive game against Australia into a laugher, going on a 19-0 run bridging halftime to turn a tight, feisty game into a 116-85 rout and advance to the semifinals of the men’s Olympic basketball tournament.

It was part of a day that couldn’t have gone better for the U.S. — not only did they win easily, but their two main rivals for the gold medal, Spain and Argentina, both suffered serious injuries that crimp whatever shot they had of upsetting the Redeem Team. The U.S. will play Argentina in the semis on Friday while Spain meets Lithuania.

Australia was hoping to crash the party, and was within seven points with 30 seconds left in the first half thanks to an overtly physical style and some stellar shooting — the same formula they’d used to frustrate the U.S. in a closer-than-expected 87-76 defeat just before the Olympics.

But the U.S. scored the game’s next 19 points to go up by 26 early in the third and cruised home from there. Unusually, 3-pointers proved the key. Deron Williams hit a bomb just before halftime to put the U.S up by 12, and Kobe Bryant made two at the start of the third quarter to leave the Aussies reeling.

In truth, it should have been a rout earlier. Australia stayed in the game partly because the U.S. missed 10 free-throw attempts in the first half and made only 2-of-12 3-pointers before Williams’s buzzer-beater; that they still finished the half with 55 points is a testament to how overmatched the Aussie defense was against the American onslaught.

Their surprise weapon was the offensive glass. Despite a size disadvantage, the U.S. had nearly as many offensive rebounds (19) as Australia had defensive boards (20); Kobe Bryant pulled in four from the guard spot and finished with a game-high 25 points.

Australia, meanwhile, adopted a defend-at-all-costs strategy to cut off the U.S. running game that other squads are likely to emulate — they didn’t even bother trying for offensive rebounds and instead just ran back as fast as they could once a shot went up.

We’ll see if Argentina tries the same thing on tomorrow, but, regardless, they’ll certainly give a better game. The defending gold medalists are a far more talented team than Australia, featuring San Antonio Spurs star Manu Ginobili and fellow NBA players Luis Scola (Rockets), Carlos Delfino (ex-Raptors), Fabricio Oberto (Spurs), and Andres Nocioni (Bulls).

Unfortunately, two major issues loom for the South American side. First is Nocioni’s knee. He appeared to be limping throughout the second half of his team’s riveting 80-78 quarterfinal win over Greece, so there’s a major question as to whether he’ll play and, if so, how well.

Losing Nocioni would be a huge blow because it would exacerbate Argentina’s biggest weakness — a lack of depth. While they have five NBA-caliber players, the Argentines’ bench is awful. This is a major problem against a team that plays as fast a pace as the U.S. and constantly subs in fresh bodies, and they’re likely to be exhausted by the second half.

Additionally, Argentina’s second-biggest weakness is the backcourt — an area the U.S. defense is likely to expose with its propensity for forcing turnovers that turn into easy baskets at the other end. Point guard Pablo Prigioni is a second-tier player in Europe, and while he’s taken good care of the ball in this tournament, he won’t be able to burn the American pressure.

That leaves most of the responsibility on the shoulders of Ginobili and Delfino. Delfino was outstanding in the Greece game, with 23 points; however, Raptors fans familiar with his Jekyll-and-Hyde history know this means he could lose to his mother tomorrow.

Ginobili, of course, is the big star on Argentina, and even more dangerous in international ball than in the NBA. The U.S. knows this all too well — he thrashed them in Argentina’s semifinal win en route to the gold medal in 2004. Kobe Bryant will get the main assignment of guarding him, with plenty of help from Dwyane Wade and the long-armed Tayshaun Prince.

The U.S. will have a major mismatch at power forward, where either Scola or Oberto will have to guard Carmelo Anthony; because of this, I expect Argentina to play zone for much of the game. Once again, 3-point shooting will be a key for the American side, but if they shoot the ball decently, their depth should overwhelm the Argentines.

If so, let’s at least throw a bone to what may be the swan song for an incredible generation of Argentine hoopsters. They’ve already won medals of all three stripes (gold in the 2004 Olympics, silver in the 2002 World Championships, bronze in the 2006 World Championships) and will probably leave here with another bronze, a remarkable feat for a group from a soccer-mad country.

However, time is not on their side. Delfino is they only key player younger than 28 and there are no players of comparable quality in the pipeline, so this may be the last we see of them on a medal stand for a long time.

In the other semifinal, Lithuania will meet Spain. The Lithuanians dispatched China 94-68 behind 23 points from Sarunas Jasikevicius and suffocating defense against Yao Ming. The Rockets’ star center looked exhausted throughout; on a positive note, this may have prevented him from killing his guards with his bare hands. China’s overmatched backcourt was its usual pitiful self at both ends, and missed Yao several times when he was open on the block.

As for Spain, they played their best game of the tournament thus far in dispatching Croatia 72-59. However, point guard Jose Calderon left late in the game with what appeared to be a groin injury, leaving his status for the rest of the tournament in doubt.

The Croats were hurt by an injury of their own. Without high-scoring guard Marko Popovic they had trouble breaking down the Spanish defense all game, frequently running the offense down to the final seconds of the shot clock. Spain started 17-year-old Ricky Rubio at point guard ahead of Calderon, and he delivered his usual mix of the spectacular and the erratic — six rebounds and a dazzling assist in 17 minutes, but also three turnovers.

Even without Calderon, I’d look for Spain’s superior skill in the frontcourt and speed advantage in the backcourt to overwhelm Lithuania. The latter club’s ball handlers are vulnerable to pressure defense, especially since Jasikevicius has lost a gear, and the speed of players such as Rubio and the Trail Blazers’ Rudy Fernandez should be too much for them.

Regardless, Argentina seems like the last remaining test for America. They’ve beaten both Lithuania (120-84) and Spain (119-82) by embarrassing margins this summer; if they meet again in Sunday’s gold medal game, the Redeem Team should roll to victory.

jhollinger@nysun.com