Anna Watkins got the better of her more decorated crew-mate Katherine Grainger when the duo competed in the singles at the British winter assessment.
Watkins, who usually competes in the doubles sculls with six-time world champion Grainger, won in a time of 19 minutes 45.55 seconds.
World bronze medallist Alan Campbell won the men's single from Tom Solesbury at the Boston Rowing Club.
Andrew Hodge teamed up with Alex Partridge to win the men's pair.
Known as the Boston Trials - and described by Campbell as "where it all begins for the Olympics" - the winter event aims to test the rowers' endurance and fitness rather than power, speed and race finesse.
In September, Britain delivered an impressive showing in the World Championships in Slovenia, winning 10 Olympic-class medals.
The squad also qualified crews in 13 of the 14 Olympic events for London 2012.
But with places in those boats yet to be confirmed, rowers were keen to impress over a 5km course, rather than the Olympic distance of 2000m, in Boston, Lincolnshire.
All rowers wishing to be considered for a place in the senior squad - and therefore be eligible for a place at the Olympics - needed to make themselves available, although leading names Pete Reed and Zac Purchase were absent through injury and illness.
With Reed recovering from a winter virus, Olympic champion Hodge rowed in a scratch pairing with world and Olympic eights silver medallist Partridge and was delighted to put in a strong showing.
"This is a great squad to be in," said Hodge. "We have so many good rowers all challenging hard for their seats in the squad. You know that you have to put in a good show here or someone else will do it and may take your seat.
"We have more testing in January, February and then the GB Rowing Senior Trials in March so we won't know for some time yet which boat we will be in next year.
"Everyone is fighting hard and wants to be the best they can be in the Olympic year."
Moe Sbihi and Alex Gregory, two strong contenders for top seats in the men's squad, looked impressive as they finished second ahead of world under-23 champions Constantine Louloudis and George Nash and veteran Greg Searle and Cameron Nichol.
Watkins, who missed a World Cup event earlier this year because of a back injury, looked to be in fine form as she eased home three seconds ahead of Beth Rodford, with Grainger five seconds further back.
Campbell lived up to his billing as the favourite in the men's single, winning the race with ease.
"Yes it's tough but it's also a good thing to do and it feels like the start of the Olympic campaign," he said.
Richard Chambers beat younger brother Peter to win the lightweight men's single scull with Rob Williams third and Olympic lightweight men's double sculler Mark Hunter, who was competing in the singles in the absence of crew-mate Purchase, fourth.
Sophie Hosking won the lightweight women's single event ahead of world under-23 gold medallist Kat Copeland.
Saturday was the first step in a series of internal and external trials, which will culminate in the senior trials on the Olympic course at Eton Dorney in March.