They finished second and first in the opening two races of the day in the two-person keelboat SKUD18 but a kit failure sent them back to the marina early in the third.
With all boats able to discard their worst score, Rickham and Birrell know there will be no margin for error with four races remaining – two on Thursday, two on Friday – before Saturday’s double-points medal race.
Natural Sciences graduate Rickham, who won bronze with Birrell at London 2012, said: “That was really unfortunate as we had gained some momentum and were really hoping to make it another top-two finish for the day.
“It would appear anything could happen in Rio but potentially it could still all be to play for. We’re going to soldier on as who knows what will happen in the last five races. The Aussies are looking strong but you just don’t know what’s going to happen.”
Meanwhile, swimmer Aaron Moores, who used to train at the University of Bath with Aquae Suils SC, led a ParalympicsGB one-two in the SB14 100m breaststroke.
Moores, whose winning time of 1:06.67 was just three-hundredths of a second quicker than team-mate Scott Quin, said: “I can’t believe it, it’s a dream come true. Scott and I train so hard, and it shows in the fact we came top two in the world.”
The 21-year-old, based at the Sports Training Village, played five Pool B games on Wednesday but was only able to pick up one win – a 5-4 victory over Italian Matteo Betti.