Kelly and Miller were roommates during their journey through the Minor Leagues and served as the best man in each other's wedding. In the days leading up to Wednesday, they exchanged texts and challenged each other.
"You think about it going into it, I guess, just knowing you're facing one of your buddies and [he] literally just got traded a couple of days ago, and then the next thing you know, he's pitching against you," Miller said. "You put that to the side. Whoever your opponent is, you're trying to beat them, no matter who it is."
When news of the trade struck Kelly in San Diego last Thursday, he found a quiet room at Petco Park, where he sat for nearly 30 minutes with a towel over his head. Kelly, who debuted with the Cardinals in 2012, had made St. Louis his home.
As he walked to the field on Wednesday for his warmup following a pregame rain delay of about an hour, the Busch Stadium crowd rose for a standing ovation. The Cardinals played his walk-up music, and another ovation ensued as he stepped into the batter's box against Miller in the third inning.
"That was crazy," Kelly said. "Definitely hard to put it in words. I got a whole standing ovation before I even started stretching. The Cardinals fans and the people here are so incredibly awesome. I can't thank them enough for that. That was awesome. My heart started beating fast. I was just trying to keep it together."
Kelly and Miller matched each other inning for inning on the mound, each taking a no-decision after surrendering one run over seven innings.
The Red Sox took the 2-1 win thanks to a ninth-inning rally, and Kelly got the upper hand in the batter's box as well. On Tuesday, Kelly told Miller if he got a hit off him, Miller would owe him $100, and vice versa.
Kelly bounced the third pitch he saw up the middle and hustled down the first-base line. He was initially called out, but a Red Sox challenge led to the overturning of the call and put Kelly on first with a leadoff single.
"He's fast," Miller said. "I've never seen him run one of those out like that. He wanted it bad, it seemed like."
And the bet?
"That was a joke," Miller said. "That was all fun and games."
"I think he messed up yesterday saying it was 100 bucks," Kelly added. "It was 100 doll hairs, from a doll's head. I'll talk to him later."
And when "later" came, Kelly emerged from the dugout and walked back to familiar territory to greet Miller on the other side of Busch Stadium.
The two slipped on their jerseys -- Kelly's the navy of the Red Sox uniform and Miller's the familiar white of the Cardinals -- and posed for a picture. A snapshot of a memorable night for two friends, reunited where it all began.