The Russians were definitely a huge part of the WWII victory but the American military industrial output was a huge part of winning the war. We were pumping out gear, ammunition, trucks, tanks, and planes at such a rate that it was staggering. Plus, once we started bombing the poop out of the German industrial areas and greatly diminishing their ability to produce military firepower on anywhere close to the level we did, it was only a matter of time. Many of the German instruments of war were much better than ours but our main advantage was our ability to produce ours in staggering numbers. I have seen interviews with German tankers in particular that talked about how their Panzers were greatly superior to the Sherman but they simply couldn't stop them all. They just kept coming. I think it would have taken a lot longer and more American lives if the Russians hadn't been on the Eastern front but our ability to out produce the Germans would have eventually won the war for us.
You also have to remember that we were fighting on two fronts as well.
http://warfarehistorynetwork.com/daily/wwii/m4-sherman-vs-german-panther/