Our London rivals - the cup holders and finalists in six of the last seven seasons - controlled the first period and took a 2-0 lead in at the break courtesy of headers from Joshua Grant and Ike Ugbo.
We produced some good play in flashes but the visitors were in command and Austin produced a string of top-drawer saves to deny all comers in the opening stages of the second half.
But as the game opened up in the last 20 minutes, we started to threaten more in the final third and found an 88th-minute lifeline when substitute Jack Roles played in Sterling to finish clinically into the bottom corner.