Britain's world champion and middleweight favourite Savannah Marshall suffered a shock Olympic Games quarter-final exit on Monday, falling to defeat in her opening bout.
The 21-year-old Briton, who won the world title in May, suffered a 16-12 upset to Kazakhstan's Marina Volnova.
After matching Marshall over the first two rounds, Volnova raised the pace as the favourite made an early exit from London, where women's boxing is making its Olympic debut.
"I am very disappointed to go out of the Olympics at the first hurdle," said Marshall.
"I have got no excuses. The preparation was right, I didn't box to my full potential."
Volnova now faces Claressa Shields of the USA in Wednesday's semi-final after the American's 18-14 last eight win over Anna Laurell of Sweden.
World silver medallist and second seed Nadezda Torlopova of Russia will face China's Li Jinzi in Wednesday's other semi after they both comfortably won their quarter-finals.
Ireland's Katie Taylor remains favourite for lightweight gold after her impressive 26-15 quarter-final demolition of Britain's Natasha Jonas.
Taylor, who faces Tajikistan's Mavzuna Chorieva in Wednesday's semi, won her fourth straight world title in May, out-classed Jonas.
"Hats off to her, there is nothing else I could have done, maybe thrown the kitchen sink or driven a bus into her," said the 28-year-old world semi-finalist.
World silver medallist Sofya Ochigava of Russia is on a collision course with Taylor for Thursday's final after hammering New Zealand's Alexis Pritchard 22-4.
It set up a semi-final clash against Brazil's Adriana Araujo, who beat Morocco's Mahjouba Oubtil 16-12 in Monday's last eight bout.
At flyweight, world champion Ren Cancan of China reached the last four with a 12-7 quarter-final win against Russia's Elena Savelyeva, but the world champion admitted feeling the strain.
"Yes, there is pressure but I am still prepared and I never look down on any opponent," said the 24-year-old who has won gold at the last three world championships.
Ren will face Marlen Esparza of the USA in Wednesday's semi after the American's convincing 24-16 quarter-final win over Karlha Magliocco of Venezuela.
In the other semi-final, India's five-time world boxing champion M.C. Mary Kom takes on Britain's Nicola Adams, who beat her in the world championships last eight in May.
Kom, 29, enjoyed a commanding 15-6 win over Tunisia's Maroua Rahali to reach the semi-finals, where the losers are guaranteed at least a bronze medal.
"I am extremely happy, now I hope my dreams come true," said Kom, who has stepped up a weight division for the Olympics and won the last of her five world crowns in 2010.
World silver medallist Adams made sure of her place with a comfortable 16-7 last eight win over Bulgaria's Stoyka Petrova.