Several fancied crews will have to fight again for the right to contest medals after a morning full of tight racing at Strathclyde Country Park.
Great Britain's top men's boat, the eight, found the Dutch pace too hot to handle in Thursday's heat and will have to race again in Friday's repechage. Their big rivals Germany, racing in heat two, made no errors and held on to qualify automatically for Sunday's final, despite a last-ditch charge from Romania.
There was also an upset in the men's single sculls as Olympic and European silver medallist Damir Martin (CRO) missed out on a semi final place. He finished third in his heat, behind Kjetil Borch (NOR) and Vladislav Ryabcev (RUS), after the Russian overtook Martin in the final quarter of the race.
Martin, who said the heat had been "just too hard", will get a second shot at the semi final in tomorrow's repechages.
The day began with the heats of the women's and men's pairs, with the leading crews from this season all progressing to the next stage of the competition. French brothers Theophile and Valentin Onfroy finished second in their heat to claim a semi final spot.
"The first race is always difficult, but tomorrow we will need good luck," Theophile Onfroy said after the race.
In the women's four, the first heat included a fight to the line between three crews racing for two spots in Saturday's final. Russia claimed the win and Poland beat Great Britain for the second place by just 0.1 seconds.
Polish stroke Maria Wierzbowska said it had taken a while for the result to sink in, especially after a slow start to their race.
"We believed right until the end and had to keep fighting. We were aware of the British boat, and the others around us, but stayed focused," she said afterwards.
In the men's four, the Netherlands and Great Britain secured comfortable wins to go directly through to the final.
Last year's lightweight men's double sculls silver medallists, Paul and Gary O'Donovan (IRL), held off a challenge from the French duo of Pierre Houin and Thomas Baroukh to win their heat.
Kristoffer Brun (NOR) and Are Strandli (NOR), bronze medallists behind France and Ireland at the Rio 2016 Olympic Games, won the first heat and world silver medallists Stefano Oppo (ITA) and Pietro Ruta (ITA) took the second, setting up a tantalising battle for medals on Sunday.
The Rowing competition at Glasgow 2018 continues on Friday, as it moves to repechages and semi finals.