Spain started World Cup as big favourites, but after a shocking 0-0 draw to Cape Verde, they corrected course with a 4-0 against Saudi Arabia. Tonight (early Saturday in European time, 2:00 AM CEST, 1:00 AM BST) they face Uruguay in Guadalajara, Mexico, knowing that they are virtually qualified for round of 32, but a victory tonight (or a draw, as long as Cape Verde doesn't score over 4 goals today against Saudi Arabia, played at the same time) would be enough to secure being Group H leaders.
And that's important, because the Group H leader will face Group J runner-up, which could be Austria or Algeria. Meanwhile, the Group H runner-up would face the Group J leader... which would be Argentina. There is, of course in theory, a big difference between having Austria or Algeria in round of 32 and having Argentina so early...
As it stands now, the most likely scenario is that Spain will face Austria or Algeria on Thursday, 2 July at 21:00 CEST, 20:00 BST, and Uruguay or Cape Verde will face Argentina on Saturday, July 4 at 00:00 CEST, 23:00 BST.
There are other possibilities beyond: as things stand now, if Spain plays against Austria or Algeria (Group J runner-up), their most likely round of 16 rival would be Portugal or Colombia, and quarter-finals would relatively easier with United States, South Korea or Egypt. Things would get complicated in semi-finals, with France, Netherlands or Germany, while Argentina, England or Brazil could wait in the final.