Hosts Brazil are still without a win at the Rio Olympic games after drawing 0-0 with tournament minnows Iraq in Brasilia.
The Selecao, led by Barcelona star Neymar, failed to break down their Group A counterparts to record a second consecutive stalemate in the men's football competition.
Rogerio Micale's men are also searching for their first goal after being held to a 0-0 draw against South Africa in their opening fixture.
Elsewhere in the tournament, Portugal and Nigeria became the first teams to qualify for the quarter-finals.
Portugal, who defeated Argentina in their Rio 2016 opener, made it two wins from two in Group D on Sunday with a 2-1 come-from-behind triumph against Honduras.
The 2008 runners-up Nigeria backed up their thrilling 5-4 win over Japan with a 1-0 victory against Group B opponents Sweden, courtesy of Sadiq Umar's first-half goal.
Germany scored late through Arsenal's Serge Gnabry to salvage a 3-3 draw with South Korea in Group C, and Japan battled from two goals down to secure a 2-2 draw against Group B rivals Colombia.
Denmark boosted their chances of progression to the next round with a tense 1-0 win against South Africa, while Argentina got off the mark in Group D with a 2-1 victory over Algeria.
Meanwhile, Fiji scored their first ever Olympic goal in their Group C clash with Mexico, but went down 5-1 to the defending champions despite leading at half-time.
The third and final group matches take place on Wednesday, with Brazil needing a win against Denmark to guarantee their spot in the last-eight.
Argentina also need to beat Honduras if they are to make it through to the quarters, while Mexico have a tough clash against South Africa.
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