Iniesta is a key component of Vicente del Bosque's squad and produced a sparklingly creative performance in Tuesday's 6-0 thumping of Poland in a friendly international before being forced off.
A scan the following day showed there was only slight swelling and no tear.
GEAR:Get your Spain shirt
"Andres is progressing very well with the minor muscular problem he sustained," chief doctor Oscar Luis Celada told television station Telecinco after Saturday's morning training session at Spain's base in Potchefstroom.
"We still don't know if he'll be fit for the Switzerland match but the feelings are positive," he added.
"There are still a few days before the World Cup starts for us and there is time to decide."
Iniesta trained apart from the rest of the 23-man squad for a second day at a sun-kissed North West University and was able to complete some more intensive ball work after light jogging around the specially-laid pitch.
Under floodlights later on Saturday and watched by several hundred boisterously-singing locals, he did more ball work and stretching as his team mates practised intricate attacking moves at the other end of the pitch.
"Little by little I am doing more and the feeling is good," Iniesta said in an interview with TVE television.
"Let's see how things go the next few days and I hope I will have a chance to play."
WEALTH OF TALENT
If Iniesta is not ready for Spain's opening match against the Swiss, Del Bosque has a wealth of talent to replace him in the starting lineup, including David Silva, Pedro and Juan Mata.
"Andres is extremely important for us," Silva said at a news conference after Saturday's morning session.
"Thank goodness he is recovering and the sooner he is back the better. He is a great player and he gives us a lot of joy."
A string of top players have been forced out of the World Cup with injury and Spain defender Alvaro Arbeloa said the squad were crossing their fingers they would be spared.
"Injuries are part of the world of soccer and all sports and sometimes you can't do anything about them," Arbeloa said at the same news conference.
"But we are 23 players and if someone gets injured we definitely have good alternatives to replace them," he added.
Xabi Alonso finished Saturday's morning training early as a precaution after suffering muscle tiredness.
He did not take part in the evening outdoor session, which was also missed by fellow midfielders Xavi and Sergio Busquets and central defender Gerard Piq
