The shotstopper denied Jamie Lowry and then Maxime Blanchard to send Oxford through 3-1 on penalties after James Constable's equaliser set up the shoot-out.
Lewis said: "Ryan's got a great record with penalties so we are always quite confident when it comes down to something like this. He's got a great presence in there, and made a couple of good saves when it counted.
"I don't think we played great. We created a few chances, they created a few chances. It was pretty even and could have gone either way really but we took the penalties well.
"James Constable made all the difference when he came on, he's got the experience and he's got the goal. We have got a busy spell coming up and so that's why he started on the bench. He's fit lad but with the game coming up on Saturday, probably half an hour was about right for him.
"I think they have got some really good young players. It's a tough game down here."