Jessica Ennis-Hill was braced for a battle for gold as she finished the first day of the heptathlon in Rio firmly on track to retain her Olympic title.
She led the field by 72 points, but team-mate Katarina Johnson-Thompson's hopes of challenging for her crown were dealt a major blow by a woeful performance in the shot put.
She was 100 points behind her compatriot in fourth place after four events.
Ennis-Hill will head into the second day of competition on 4,057 points, 101 down on the same stage at London 2012.
With three more events to come on Saturday - the long jump, javelin and 800 metres - the 30-year-old is well-placed to make history by becoming the first British woman to retain an Olympic title in athletics, some achievement two years after the birth of her son Reggie.
"It's always nice to be leading after the first day. But those girls have got big jumps in the long jump and they can all run a good 800m, so it'll be a challenging day tomorrow," said Ennis-Hill.
But one of them, Canada's world number one Brianne Theisen-Eaton, was struggling as she lay in sixth place overall, 186 points off the lead.
A repeat of London 2012's 'Super Saturday' is on the cards after Greg Rutherford held his nerve to qualify for the long jump final, but the mouth-watering prospect of a titanic tussle for gold between the two British heptathletes, which would have provided one of the highlights of the Games, looks unlikely to materialise.
Johnson-Thompson, the heir to her team-mate's multi-eventing throne, is still very much in medal contention, though, trailing second-placed Nafissatou Thiam of Belgium by only 28 points.
"It's not over," she said. "I can't give up on my dream."
She could, though, have been so much better placed but for a dreadful shot showing.
The 23-year-old could only manage a best throw of 11.68m. The event is the Liverpool athlete's weakest by far, but this was still a well below-par performance.
"With the shot I just lose my head a little bit, there's no excuse for it, but I add that into my score now as though it's expected," she said.
She rectified the situation by clocking the fastest time in the 200m, her 23.26 seconds edging out Ennis-Hill's 23.49secs, but was still left playing catch-up on her team-mate.
Ennis-Hill had to settle for a best throw in the shot of 13.86m and admitted she was "devastated" with the effort after throwing around 14.50m in training.
The day started so promisingly for the British pair - and Johnson-Thompson in particular after she produced a British record clearance - and equal world heptathlon best - of 1.98m in the high jump.
It was some statement of intent from an athlete who was left distraught at last year's World Championships in Beijing when three fouls in the long jump dashed her medal hopes. She will now be hoping her shot woes have not done the same.
Her team-mate, the defending champion, also got off to a flying start, clocking 12.84s in the 100m hurdles and then clearing 1.89m.
It was her third-fastest time since London 2012 - and her fastest in a heptathlon since then - and her best high jump since before the last Olympics.
Ennis-Hill declared herself "really pleased" with her hurdles and high jump.
Johnson-Thompson had clocked 13.48 over the hurdles, a solid enough time given the rainy conditions at the start of the morning.
The Britons were hit by a wall of noise on their first morning of London 2012, a teenage Johnson-Thompson saying 'wow' at the roar which greeted her introduction.
There was no chance of that at a near empty Olympic Stadium for the start of competition here, even if the British fans were the ones making the most noise.
There looked to be barely a couple of thousand spectators in the 56,000-capacity venue when the action got under way in the morning and at the start of the evening session it was less than a fifth full.
Laura Muir, fresh from breaking Kelly Holmes' British record on her last appearance, breezed into the 1500m semi-finals, taking her heat easily to go through in third place in 4:06.53.
Laura Weightman had to rely on a fastest loser spot to advance, though, while Ireland's Ciara Mageean also progressed.
Matthew Hudson-Smith booked his spot in the 400m semis, but there was agony for European champion Martyn Rooney as he crashed out.
Desiree Henry looked impressive in winning her 100m heat in 11.08. Asha Philip also went through, but Daryll Neita crashed out.
Michael Rimmer and Ireland's Mark English qualified for the 800m semi-finals, but not Elliot Giles.
Sophie Hitchon qualified for the hammer final.
Meanwhile, American Michelle Carter beat New Zealand's two-time Olympic champion Valerie Adams to shot put gold.