"I think the key was to go out there aggressive," Diaz said. "I looked at the ball and realized [Matt Holliday] was playing a little back and I had to make the catch."
With his team already up, 6-0, in the second inning, Cubs third baseman Kris Bryant singled with one out to set up Diaz's best moment in the field this season. Anthony Rizzo shot a ball skyward into shallow left, and Diaz darted back toward a potential landing spot.
"I like how he's going back," Cardinals manager Mike Matheny said. "He was a little more tentative early, and now you see him much more in control and aggressive."
As he tracked the ball back over his head, Diaz slid and swiped it out of the air. Bryant didn't think Diaz had a chance at a play and was already around second base when the catch was made. Diaz threw the ball to Kolten Wong, who relayed it to Matt Adams, doubling Bryant at first base and completing the irregular six-four-three double play.
Statcast™ numbers illustrate the incredible catch. Diaz took his first step in 30 seconds and covered 91.9 feet, with a route efficiency of 97.6 percent while reaching a top speed of 20.2 miles per hour.
"He is really, really making strides there at shortstop," starter Michael Wacha said. "He's a heck of a player for us. That was a big double play."
Even the victim of the highlight-reel moment praised Diaz.
"It was a really, really nice catch," Rizzo said. "It stinks it was on me, but you've got to appreciate a play like that."
Though the Cardinals still lead the Majors in errors, with 41 -- including two on Tuesday -- at least one player could gain some confidence.
"It felt good," Diaz said. "Every game I feel more comfortable and ready to play on this level."
Nick Krueger is a reporter for MLB.com based in St. Louis. This story was not subject to the approval of Major League Baseball or its clubs.