This quarter-final sees the six strongest chefs from this week's heats attempt to impress Monica and Marcus in two high-stakes challenges. First up the chefs face a brand new challenge - the judge's favourite ingredient test. The six chefs are challenged to invent a dish centred on one key ingredient. This week, Marcus has chosen for the chefs to elevate a great staple of any kitchen - the potato. With a whole host of varieties and a never-ending list of ways to cook the potato Marcus is hoping for ingenuity, elegance and above all great tasting plates of food from these chefs. The chefs have ten minutes to choose from a range of ingredients and 70 minutes to cook a plate of food that delivers the humble spud as the star of the show. Only the four stand out chefs go on to the next challenge, to cook for the restaurant critics - for two chefs it is the end of the competition.The remaining four professionals go on to cook their food for three of the UK's most discerning restaurant critics. They need to impress Tracey MacLeod, Tom Parker Bowles and Jay Rayner. The critics dish up their insight and unrestrained opinion on the food. With only one hour and 15 minutes to cook a two-course menu designed to dazzle the critics and the judges, the chefs battle to remain calm in the kitchen. Only the three most talented chefs will go through to Knockout Week and continue their pursuit of the MasterChef: The Professionals 2018 title.