Tiffany Porter qualified for the semi-finals of the 100 metres hurdles at the London Olympics this morning, but did little to ease fears she is not fully fit.
Porter could only manage third in her heat in 12.79 seconds, 0.08secs behind winner Alina Talay of Belarus.
She always looked sure of the top-three finish needed for progression, but appeared to be running close to flat out.
The 24-year-old was an injury doubt coming into the Games after she left the track at last month's Diamond League meeting at Crystal Palace in tears, a back injury having flared up.
United States-born Porter is one of the athletes in the team who has been dubbed a 'Plastic Brit' by critics, but not by the vast majority of the crowd judging by the roar which greeted her introduction.
She lost her British record to Jessica Ennis in her stunning first event of the heptathlon on Friday.
The newly-crowned Olympic champion did not line up in the heats, having stuck with her decision not to compete in the hurdles.
The race was delayed by a clear false start, which saw Mali's Rahamatou Drame disqualified.
Porter insisted she was satisfied with the run.
She said: "It was very comfortable. The objective was just to qualify and get through to the next round and I did that.
"I have been very anxious and nervous but I am fine and very confident for the rest of the competition."
On the back injury, she said: "It's fine, I'm very confident and I will go out there and do my very best."
Hot favourite Sally Pearson, the Australian world champion, was the most impressive qualifier, clocking 12.57s, the fastest first-round time in Olympic history.
But Jamaican Brigitte Foster-Hylton, second in the world rankings, was a major casualty as she clattered the fifth hurdle and trailed home seventh.
Wales' Brett Morse crashed out in discus qualifying, registering only one legal throw of just 58.18m, way down on his season's best of 64.35m.
Morse said his performance was "very disappointing really".
He added: "In the first round I walked in and wound up and the crowd just went mental so I was a little nervous on that throw. I didn't actually hit any of the throws properly.
"I think I just rushed the finish, I was a little excited maybe, I don't know."
Andrew Osagie, the world indoor 800m bronze medallist, lived very dangerously as he claimed the third place needed to progress in his race by 0.03secs.
In a race controlled by world record holder and gold medal favourite David Rudisha, Osagie left himself plenty of work to do in the home straight, just managing to come through before the line to finish in 1:46.42.
Asked if he was breathing a sign of relief, Osagie said: "A little bit. The heats are always nervous. I am renowned for running rubbish in the morning.
"I was in a really relaxed position. I just wanted to settle down, with this sort of crowd it's easy to get out of control and get an adrenaline rush.
"I wanted to use them as a barrier and wait until the last 120 metres.
"The kind of performances the guys put in at the weekend means if you don't run well you're going to be the laughing stock of the team."
Gareth Warburton, who won selection on appeal after initially being left out of the Olympic team, went out, finishing fifth in his heat in 1:46.97.
There was also disappointment for Michael Rimmer, who completely ran out of steam in the final 50 metres of his heat to finish fifth in 1:49.05, more than four seconds down on his best this year.
Rimmer, who has struggled with injury since winning European Championship silver two years ago, said: "I have no answer to be honest, shocked.
"My legs did not have the change of gears. I am really confused because I was running PBs in Portugal (at the pre-Games training camp).
"I was absolutely smashing it in Portugal. I don't understand, it's upsetting."
Warburton, who was competing in his first major championships, said: "I was in a good position at 600m and they just ran away from me in the last 150m. That is 800m running.
"It is an Olympic Games. Every heat is tough.
"I take experience from this. I have learned a lot, not just from running here today but from the whole run-up."
All three of Britain's women's 1,500m runners advanced to the semi-finals.
World Championship silver medallist Hannah England always looked well placed in her heat.
England, whose build-up has been hampered by a spiked Achilles tendon she suffered in a race in May that kept her out for around six weeks, came home fifth in 4:05.73.
Months after being diagnosed with potentially fatal blood clots on her lungs and told to give up on the Olympics, Lisa Dobriskey showed she was getting back to her best by winning a slow heat in 4:13.32.
Laura Weightman, who is trained by former world record holder Steve Cram, finished sixth in the third heat in 4:07.29.