Both of these would have made a huge difference.
If Great Britain had fallen, it would have been almost impossible to invade Western Europe. There would not have been this safe haven where the US could bring all its equipment and from which it could bomb Europe. So Western Europe would have been solidly held by Germany.
If the USSR had fallen, so much of Germany's army could have moved over to the west to defend that area and D-Day could probably not have succeeded. The Russians killed more Germans than anyone else -- if they had been out, things would have been way easier for Germany.