The United States didn't land in Africa until 1942. By this time, Germany was already not doing so well on the Eastern Front and in Africa against the British. Without American supplies then yes, Germany would likely have won. However Germany and its European allies had already suffered over a million casualties by the time America entered and began fighting. Even without the United States military in Europe, Germany would have lost at Stalingrad and Kursk and the war would most likely have been won. The British could have pushed out Germany alone since they were capable of defeating Rommel at El Alamein.
By the time the UNited states entered the war, Germany was in hot waters, but the only reason it was in a bad spot, was because the US was the logistical backbone for all sides opposing Germany. Without US support Nazi Germany would have won with little effort. It was US food, medicine, military equipment, weapons that stem the tide.