"I told him, `Really? You're going to swing first pitch like that?" Fernandez said. "That swing had a lot of bad intentions."
It was a sign of things to come. Diaz homered, doubled and drove in three runs in the first-ever meeting between the two childhood pals from Cuba, helping the Cardinals beat the Miami Marlins 5-4.
Fernandez fell behind 5-0, departed after five innings and dropped to 26-2 at Marlins Park.
"I made some mistakes and they took advantage. That's on me," he said. " Every time you get four runs and have your ace on the mound, you should expect to win."
Miami's Dee Gordon, the 2015 NL batting and stolen bases champion, returned from an 80-game suspension for failing a drug test and went 0 for 4. Ichiro Suzuki doubled as a pinch hitter in the seventh for Miami and needs two hits for 3,000.
Diaz and Matt Holliday homered in the third inning against Fernandez (12-5), who had never previously given up more than one homer in a home game.
"That was a lot of fun to compete against my buddy," Diaz said. "I just tried to be aggressive with him, he's such a great pitcher."
Fernandez's only other loss at Marlins Park came on opening day this year against Detroit.
Diaz and Fernandez were neighbors growing up in Santa Clara, Cuba, and the Cardinals enjoyed their reunion. Fernandez walked Jeremy Hazelbaker to start the third inning, and Diaz followed with his 14th homer.
"This guy wants to hit it 10,000 feet," Fernandez said. "He's helping his team win. You can't get mad about that."
Two batters later, Holliday hit his 18th homer just inside the right-field foul pole, prompting a rueful grin from Fernandez. The outing was his shortest since May 4.
The Cardinals homered in 17 consecutive games before being blanked in that department Wednesday.
Hazelbaker tripled and Diaz drove in a run with a double, his 25th, in the Cardinals' two-run fifth.
Marcell Ozell hit comebackers off Wacha (right foot) and Oh (left buttocks). Both stayed in the game. It's the third game in a row Wacha has been hit on the right foot.
"It's got a bull's-eye on it," Wacha said.
The umpires said they erroneously reversed a balk call, taking a Cardinals run off the board in the second inning.
With runners at second and third and two out, Fernandez was about to intentionally walk No. 8 hitter Kolten Wong when third base umpire D.J. Reyburn called a balk. Fernandez protested, and after the four umpires conferred, they reversed the call and decided there was no balk.
Wacha struck out to end the inning and keep the game scoreless. After the game, crew chief John Hirshbeck reviewed video and decided Reyburn's balk call had been correct.
"D.J. called it from third, and I thought I saw (Fernandez) step off with his right foot," Hirshbeck said. "I overruled D.J. with what I thought I saw. He was right, I was wrong."
Suzuki received a standing ovation when he pinch-hit, and he responded with his 350th career double, raising his season average to .335. He has started only one of seven games on the homestand.
"I don't think he's searching to make this about him," Marlins manager Don Mattingly said. "I think he's comfortable with the way we're playing it."
RHP Lance Lynn, who underwent Tommy John surgery in November, threw to hitters again Thursday, and the Cardinals haven't ruled out his return this season.
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