Wales fly to New Zealand a day after taking on South Africa and will play Tests on 19 and 26 June. There is also a possibility Wales will kick off their trip down under with a clash against New Zealand Maori on 12 June.
Lewis said: "This [the clash with South Africa] will undoubtedly be the showcase rugby occasion of the summer in the northern hemisphere and is a great coup for Wales.
"The Millennium Stadium crowd will also be able to give Wales a rousing send off for their summer tour to New Zealand.
"We are now seriously building towards next year's Rugby World Cup and Welsh fans can be certain that our international fixture list from now on will fully reflect the importance of that process.
"Warren Gatland wants his team to face the best and we have worked tirelessly to ensure we deliver on that wish in the months ahead."
WRU chairman David Pickering, who was Wales manager when Graham Henry's side beat New Zealand in 1999, said: "The timing of the game is excellent in that it gives our team a huge boost before they set off for New Zealand.
"It will be a landmark day for Welsh rugby and a unique opportunity to see some great rugby action in the northern hemisphere at that time of year."
Pickering also thanked South Africa Rugby Union president Oregon Hoskins for agreeing to the fixture.
Hoskins accepts the fixture's timing is not in the Springboks' best interests, but says Saru had no choice after it was "begged" by the WRU to play the game.
"We acknowledge that the clash is untimely given the tough Bok schedule in the build-up to the World Cup," the Saru president told South African publication Business Day.
"But the Welsh Rugby Union begged us in honour of the Test 10 years ago in which the Boks opened up the Millennium Stadium.
"It means a lot to them and we didn't want to let them down, even though we admit it is not in our best interests."
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