Nigeria 2-0 Iceland
Leicester City striker Ahmed Musa struck twice to guide Nigeria home and keep everyone's hopes alive in an intriguing Group D.
Previously, Croatia had already qualified for the round of 16, but Iceland had a grand opportunity to stake their claim, after drawing with Argentina in their tournament opener.
Those hopes were shattered, when Musa opened the scoring in the 49th minute and then confirmed an African win in the 76th minute.
"What I like in my young team is himility, solidarity and fighting spirit," said Nigerian coach Gernot Rohr. "I thought before this World Cup we were here to learn.
"I think this team will be ready in 2022 and this World Cup is coming early, but we have a good chance to win against Argentina."
Fan favourites Iceland had a chance to close the margin with 10 minutes remaining, but Gylfi Sigurdsson lifted a penalty attempt ineffectually over the crossbar.
"It's a strange fact that we are still in the race," said Iceland coach Heimir Hallgrimsson. "We didn't think we would go through this World Cup without a loss and losses are always difficult to swallow, but we can still go through."
Nigeria have now put themselves in the driver's seat for the second qualifying spot out of the group and can move on with a result against Argentina next Wednesday. Iceland will need a win over the impressive Croatians to keep their hopes alive.
Brazil 2-0 Costa Rica
Brazil needed two injury-time goals to sneak home against their Central American neighbours, putting them on the verge of progressing to the knockout rounds.
The Costa Rican underdogs kept the flaccid Brazilians at bay for most of the match, before Philippe Coutinho and Neymar finally decided the contest with goals in the 91st and 97th minutes respectively.
Brazil outshot their rivals 23-4, with nine of those on target.
"It was a tough game as we expected," said Coutinho. "We showed good patience to play until the last minute, where we rewarded with those two goals.
"We know the responsibility is huge when you'r eplaying for Brazil. Emotionally, the professor [coach Tite] always highlights the mental strength we need from the start to the end."
The result puts the underwhelming South Americans in the box seat in Group E, after their opening draw with Switzerland, but it spells an early exit for Costa Rica, who fell to Serbia first up.
Brazil will face Serbia on Thursday, with Switzerland also still a chance to qualify.
Switzerland 2-1 Serbia
Xherdan Shaqiri's goal at the end of the regulation time was enough to put the Swiss ahead for good and throw Group E into disarray.
Serbia seemed to have this game in the bag, when Aleksandr Mitrovic scored after five minutes with a header that found the back of the net.
But as the game wore on, slowly Switzerland took the upper hand, drawing even with a thunderbolt from Granit Xhaka in the 52nd minute. Shaqiri's initial shot was blocked, but Xhaka was on hand and gave Serbian keeper Vladimir Stojkovic no chance.
The contest seemed headed for a draw, until Shairi cooly slotted a losse ball past Stojkovic for the winner in the 90th minute.
The result leaves Brazil and Swtizerland, who fought out an opening draw, on top of the group, but Serbia lurks just a point behind.
Anything can still happen, but the Swiss appear well placed, with just the winless Coast Ricans between them and the round of 16.
The Brazil vs Serbia showdown may prove the crucial game to decide who progresses.