Marshall, a silver medallist two years ago, needs to again reach the semi-finals in Qinhuangdao to guarantee her London 2012 place but had a nightmare draw against highly-rated and previously undefeated US teenager Claressa Shields.
Shields has collected a string of top scalps, winning the Pan-Am Games title last year, but Marshall proved too wily and too strong, winning 14-8.
However, it doesn't get any easier for the 21-year old from Hartlepool, who will now face Norway's European champion Lotte Lien.
Elsewhere, lightweight Natasha Jonas won her second bout to progress to the last 16, meaning she is just two victories from earning selection. Liverpool's Jonas was a confident 23-11 winner over Switzerland's Sandra Brugger.