I'm not so completely conviced yet about Hewitt's play, for sure a more than solid comeback after being out of competition so long, but if Karlovic is serving well he could make it very tough for Hewitt, maybe even pull out a win. It's not impossible for him to get a few return points off Hewitt in tiebreaks to win these if he can hold his own serve very well.
Then again, Karlovic seemed to enjoy a much better time last year and I also don't rate Karlovic that good at the moment despite winning matches on grass and actually breaking serve. But I have to say that I didn't see any of Karlovic's matches on grass so far.
I surely wouldn't feel comfortable being forced to bet on this match
If I HAD to bet money, I would trust Hewitt because I have the feeling/hope it might be the winning side at the end.