Splitting the pair is Murray's compatriot Katy Burke, who picked up 80 penalty points after she put down four fences at Rostov's hippodrome.
Marques is well-placed for a tilt at her first World Cup gold as she begins the combined event in third having knocked down three fences. And Victoria Tereshuk, who sets off sixth, is capable of making up much bigger deficits than 18 seconds when she is on form with her running and shooting.
Olympic silver medallist Heather Fell (GBR) lies in seventh going into the combined, four places ahead of Anastasiya Prokopenko (BLR) who won this morning's fencing.
A number or competitors were unseated during the riding event, including the winner of the swimming event Gulnaz Gubaydullina (RUS) who needed extensive medical treatment.