Reading looked set to add further momentum to their late-season run at the play-offs as they led late into the game thanks to Gylfi Sigurdsson's sixth-minute opener.
Baggies goalkeeper Scott Carson made some vital saves to keep his side in the contest and was rewarded when Jonas Olsson's shot rebounded kindly off the woodwork for Tamas to fire an 86th-minute equaliser to leave the visitors a point behind leaders Newcastle.
"We knew it was going to be difficult for us. A point here is a very good point and we had to chase the game for most of it," Di Matteo said.
"This was very exciting. We have had quite a few of those games where we battled away and got something out of it.
"Scott Carson was top drawer. He kept us in the game. He was brave and everything you look for in a goalkeeper."