Olympic champion Kat Copeland made a winning return to competition after almost two years out, winning her European Championship double sculls heat alongside Imogen Walsh.
The 23-year-old took a break after London 2012 and is competing in her first major regatta since.
"I am glad to get it over with really. I was a bit nervous," she said.
Great Britain's men's four set a European best time of five minutes and 49.08 seconds in winning their heat.
Andrew Triggs Hodge and Alex Gregory, London 2012 gold medallists in the event, have been paired with Moe Sbihi and George Nash and together they brought their boat home more than a second and a half ahead of Greece in second.
Women's pair world champions Helen Glover and Polly Swann are also through to their final after beating the Dutch duo of Aletta Jorritsma and Heleen Boers by almost four seconds.
Copeland, who won a shock gold with Sophie Hosking in London, has said that she intends to defend her Olympic title in Rio.
She and Walsh are two of 50 GB rowers across 17 boat classes in the three-day regatta, which goes ahead despite flooding in Serbia.