Andrew Osagie claimed the third place he needed to progress to the semi-finals of the 800 metres.
In a race controlled by world record holder and gold medal favourite David Rudisha, Osagie left himself work to do in the home straight, just managing to come through before the line to finish in one minute 46.42 seconds.
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 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."