Natalia Sheppard beat British team-mate Sophie Troiano 12-9 in a really tense opening women's foil contest in the Olympic fencing at the ExCeL Arena.
Ranked 56 places above her opponent at 42nd in the world, Polish-born Sheppard went 6-2 up midway through the fight, but Londoner Troiano got the deficit back to one point on no fewer than five occasions.
Sheppard, who came through a European qualifier to earn her spot, refused to let her opponent get back on level terms, however, and two hits in the final six seconds took her through to a second-round contest against France's world number six Corinne Maitrejean.
A relieved Sheppard said: "I was so scared, so nervous - the first fight is the hardest because you're always so worried about failing at the first hurdle. It's so difficult mentally facing a team-mate and she's fencing so amazingly. It's a good sign for the team event."
Troiano said: "I wish I'd been more switched on at the start, but you're bound to be nervous - it's the greatest show on earth. I'll regroup for the team event. That's where the real focus is."
There was real heartbreak for Anna Bentley, Britain's other woman foilist - she was one second away from pulling off a huge upset over Canadian Monica Peterson, but had to go into sudden death and lost 10-9.
Like Troiano, Bentley was given a host nation spot and at 165th in the world was ranked 122 places below her opponent. The 31-year-old Aberdeen-born fencer, who took up the sport at school in Norwich, led 3-1 early on, then found herself 8-6 behind, but scored the next three points.
There was huge roar when she went 9-8 ahead with only six seconds remaining, but Peterson's late lunge kept her alive and she struck again with 37 seconds remaining.
"The crowd really helped and I thought it was mine, but you have to die by the referee's decision," said Bentley, who believed she might have been given the decision when the two came together.
"Competing in the individual was a real bonus for me. I was chosen because of the team event and hopefully now we come together after Natalia and Sophie had to fight each other."