Cape Town - Eoin Morgan mused that England had taken a backward step after failing to get over the line against South Africa in the first Twenty20 but heaped blame on himself for the dramatic one-run defeat.
Jason Roy lit the touchpaper to a chase of 178 with a blistering 70 from 38 balls but England lost their way after his departure with 46 still required from the final 34 deliveries under the East London floodlights.
Morgan put England back in the driving seat with some thunderous blows in the penultimate over to leave them needing seven from as many balls, but he then immediately holed out to long-on for 52 to expose the lower order.
After England were unable to hold their nerve in the final over, Morgan was rankled by his dismissal but felt there was a wider malaise in the performance of a team that fielded nine members of their World Cup-winning squad at Buffalo Park.
"It was in the absolute slot to check down the ground and I've smoked it straight to the man," the England captain said. "I was fuming with myself, I hit it way too well.
"It's a mistake, it should never have happened. I should have been facing coming into the last over with six to win.
"But when you lose a game by one run, there are many things you can look at. For us it is a matter of that because the way we played wasn't up to our normal standard, it's almost like it's gone back a bit and I don't know why."
The equation came down to three off the last ball after Tom Curran had heaved straight down the throat of deep midwicket and Moeen Ali had been cleaned up, the pair foxed by Lungi Ngidi's excellent death bowling.
After Adil Rashid was run out coming back for a second that would have tied the game, Morgan hailed the South Africa paceman's off-cutters in the 18th and 20th overs as defining moments.
However, Morgan still feels his side, which has aspirations to win the T20 World Cup later this year in Australia, should have been able to get home in that last over with only seven runs required.
"Absolutely, nine times out of 10," Morgan added. "But normally when there's not a guy with that sort of skill level of a slower ball, you win that game.
"It's another challenge for us, we'll have to come back and come up with a counter plan or try to target somebody else."
England struggled to contain South Africa after inviting them to bat first in the setting sun as openers Temba Bavuma (43), Quinton De Kock and Rassie van der Dussen (both 31) took the Proteas to 105 for one at halfway.
However, after Moeen had finished with one for 22, fellow spinner Adil Rashid took one for 23 to put the breaks on South Africa, who lost four wickets for eight runs in the last two overs to finish on 177 for eight.
Roy's 22-ball fifty - England's joint second fastest in this format - got them off to a flyer while there were cameos of 15 and 23 from Jos Buttler and Jonny Bairstow.
However, after Joe Denly had sent a catch to deep midwicket, Ben Stokes did likewise off Ngidi, who finished with three for 30 to help South Africa draw first blood in this three-match series which resumes at Durban on Friday.
With veteran Dale Steyn returning to international cricket for the first time in 11 months and Kagiso Rabada rested from this series, Ngidi knows he must deliver because of the healthy pace stocks available to South Africa.
"You've got to be on your A-game," he said. "If you're not quite there, someone else is going to come in. I don't plan on letting anyone take my spot so I'm just going to keep playing as best as I can."