With the victory in the winner-take-all American League wild card game, the Rays booked a Division Series clash with the Houston Astros.
Diaz, who returned at the weekend after being sidelined since late July with a broken left foot, homered to lead off the game and homered again in the third inning.
The offensive support was more than enough for Rays starting pitcher Charlie Morton, who struck out four in five solid innings.
Morton needed 32 pitches to get through the first inning, but he got Jurickson Profar to fly out with the bases loaded to end the threat.
Morton worked his way out of another couple of jams, allowing just one unearned run as he became the first pitcher in MLB history to win three winner-take-all games.
"You know what, the first inning I kind of said to myself we're lucky to have Charlie Morton on the mound," Rays manager Kevin Cash said.
"Because we get the 1-nothing lead a young pitcher in that situation, that environment, how loud it got, you just wonder how he's going to handle that.
"But Charlie, he's a veteran. Charlie wasn't at his best today, but he certainly made his best pitches when they counted the most."
The Rays were undaunted by a crowd of 54,005 -- a record for an MLB wild card contest.
Diaz -- who started at first base after speculation he'd be limited to a designated hitter role -- became the first leadoff hitter in MLB history to homer in each of his first two at-bats of a single post-season.
Things only get harder from here as the Rays tackle the Astros, the 2017 World Series champions who won an MLB-leading 107 regular-season games.
"Look, they're better than us. Their pitching is better, their hitting is better," Rays principal owner Stuart Sternberg admitted before Wednesday's game as he looked ahead to a possible match up with Houston.
"I feel pretty strong that over a five- or seven-game series we're going to be involved in a bunch of one-run, two-run games, and those games you can be on either side of it."