"He probably wasn't talked about going until about two weeks ago, he's had a tremendous season," Williams added.
"He's only 20 years old so I'm real glad to see Leigh there and looking forward to seeing him wear that red shirt for the Lions.
"In 12 months he's gone from playing rugby for Cardiff [RFC] to the Cardiff Blues, Wales Under-20s to Wales and now Lions selection.
"It's incredible and it's great for him, this really will make him as a player - he's a great player now and it just makes you think what he'll be like in a couple of years."
Halfpenny is the youngest in the squad for South Africa. The next task in his meteoric rise will be try to claim a starting Test spot for the Lions, with Williams, Ugo Monye, Luke Fitzgerald - only 21 himself - and Tommy Bowe the other specialist wings in the squad.
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