Great Britain produced a battling performance at the Basketball Arena - but it was not enough to prevent an opening game defeat against reigning Paralympic wheelchair rugby champions the United States.
Britain, inspired by free-scoring star man Aaron Phipps, captain Steve Brown and workaholic David Anthony, led 13-11 after the first quarter.
But they struggled to contain America's slick, impressive movement off the ball, and once the gold medal favourites went ahead early in the second quarter they gradually pulled away.
Britain, fourth at the last Paralympics in 2008, have remaining Pool A games against France tomorrow and then Japan on Friday, when victories in both should secure a semi-final spot.
Wheelchair rugby, known as murderball when it was invented in 1977 due to the ferocity of collisions between players, did not disappoint a capacity crowd on its London Paralympics debut.
Mayor of London Boris Johnson was among a crowd often left wincing by the hits as both teams looked to lay down a gold medal marker.
Brown scored the tournament's first goal, and his team rapidly followed the skipper's example during an opening quarter high on energy and endeavour.
Britain shaded it by two goals, staying ahead throughout, and America did not go ahead in the match until almost three minutes of the second quarter had elapsed.
The defending champions eased through the gears, opening up a five-goal advantage with the brilliant Chuck Aoki in commanding form, even though goal machine Phipps maintained a prolific strike-rate.
America took a 41-32 lead into the final quarter, and that was too big a margin for Britain to have any realistic chance of staging a successful fightback.
Reflecting on the defeat, Brown said: "United States are the number one team in the world, but I am so proud of my players.
"The atmosphere was something else today, and we drew from that as players.
"In so many areas we matched up. Player for player, there is very little difference.
"They are a very well-oiled machine and they have played together for a long, long time."
United States coach James Gumbert paid tribute to the British performance.
"We played a quality opponent today.
"They came out and threw everything in the tool-bag at us, and we had to go back and find another tool-bag.
"They are a quality team and they put on a quality performance. They played as champions."