Wales were forced to overturn a half-time deficit as they battled to an unconvincing 22-18 victory over Japan in the first Test in Osaka on Saturday.
The visitors were oustcored two tries to one but a big second half saw them come from behind to snatch victory in the last quarter.
Wales hit the front for the first time on the hour mark having struggled at the breakdown and the set-pieces against a Japanese side seeking their first victory over a top-ten nation.
The hosts led 11-6 at the interval but couldn't match the strength of the Welsh bench as they faded in the latter stages of a hard-fought battle in temperatures in excess of 30 degrees at Hanazono Field.
Wales were without 15 senior players on duty with the British and Irish Lions in Australia, and a number of experienced names left out of the touring party, and it soon became apparent they would miss the presence of a genuine openside flanker.
Wales were on the back foot for much of the opening quarter and trailed 6-0 as Japanese full-back Ayumu Goromaru landed his first two shots at goal while Welsh number eight Rob McCusker was sent to the sin-bin.
Dan Biggar got Wales on the scoreboard as the visitors began to find some rhythm and the fly-half levelled the scores around the half-hour mark.
But it would be Japan who would cross the whitewash first as New Zealand-born flank Michael Broadhurst ghosted over on the stroke of half time after a line-out move.
Goromaru missed the conversion but Japan nevertheless went into the changing rooms with a five-point lead.
Japan still looked the better side after the interval, but Goromaru missed two penalty attempts.
Biggar landed two penalties in the second half to edge Wales ahead before wing Harry Robinson finished off a wonderfully-worked try - including a superb offload from full-back Liam Williams - to give the Six Nations champions some breathing room. Biggar converted.
But it would be a close finish as Japan came storming back. A series of drives up the middle eventually created space on the right for wing Yoshikazu Fujita to cross on the right.
Goromaru's conversion made it a one-point ball-game at 19-18 going into the final ten minutes.
But replacement fly-half Rhys Patchell would have the last laugh for Wales, slotting a 79th-minute penalty to secure the win.
The teams face each other again in Tokyo on Saturday, June 15.
Tries: Broadhurst, Fujita
Pens: Goromaru 2
Pens: Biggar 4, Patchell
Yellow card: McCusker
Japan: 15 Ayumu Goromaru, 14 Yoshikazu Fujita, 13 Male Sau, 12 Craig Wing, 11 Kenki Fukuoka, 10 Harumichi Tatekawa, 9 Fumiaki Tanaka, 8 Takashi Kikutani (c), 7 Michael Broadhurst, 6 Hendrik Tui, 5 Shoji Ito, 4 Hitoshi Ono, 3 Hiroshi Yamashita, 2 Shota Horie, 1 Masataka Mikami.
Replacements: 16 Takeshi Kizu, 17 Yusuke Nagae, 18 Kensuke Hatakeyama, 19 Toshizumi Kitagawa, 20 Ryuta Yasui, 21 Atsushi Hiwasa, 22 Yu Tamura, 23 Hirotoki Onozawa.
Wales: 15 Liam Williams, 14 Harry Robinson, 13 Owen Williams, 12 Jonathan Spratt, 11 Dafydd Howells, 10 Dan Biggar, 9 Lloyd Williams, 8 Rob McCusker, 7 James King, 6 Andrew Coombs, 5 Lou Reed, 4 Bradley Davies (c), 3 Scott Andrews, 2 Emyr Phillips, 1 Ryan Bevington.
Replacements: 16 Scott Baldwin, 17 Rhys Gill, 18 Rhodri Jones, 19 Andries Pretorius, 20 Dan Baker, 21 Tavis Knoyle, 22 Rhys Patchell, 23 Tom Prydie.
Venue: Hanazono Field, Osaka
Referee: Lourens van der Merwe (South Africa)
Assistant referees: Greg Garner (England), tba (Japan)
Assessor: Lyndon Bray