This story was excerpted from Daniel Kramer’s Mariners Beat newsletter. To read the full newsletter, click here. And subscribe to get it regularly in your inbox.

ST. PETERSBURG -- Julio Rodríguez beamed that megawatt smile after making the running catch, and he didn’t slow his sprint when racing toward the dugout to ensure his victim caught a glimpse of his playful reaction.

That’d be Noelvi Marte, the former Mariners top prospect whom Rodríguez this week in Cincinnati called “my brother.” It was a fun moment that illustrated the affable personality of the Mariners’ star -- but from a pure baseball standpoint, it also highlighted the center fielder’s blossoming skill.

Rodríguez covered 49 feet in 3.5 seconds to make the grab, converting a play that Statcast projected with a 65% catch probability. It was hardly his flashiest defensive moment this year -- the home run robbery of Fernando Tatis Jr. last month will be hard to top -- but it was one he was perhaps most satisfied with because it contrasted to what’s been a specific challenge: running in on balls.

“I feel I've improved a lot more,” Rodríguez said. “Like, my reads, everything, I've probably improved a lot. Maybe a lot of people don't talk about it, but I feel proud of it. I'm going to keep taking pride in it. All my teammates know that, and they feel secure out there in center field.”

Rodríguez entered this series at Tampa Bay ranked third among 126 qualified outfielders with 11 outs above average. Yet the one area where he grades negative is when running in, where he’s worth minus-1.

Those plays are typically the most challenging for any outfielder. Rodríguez is only 22, only in his second season and only shifted to center during the final month of his final Minor League season in 2021 -- the point being that the Mariners believe there’s plenty of room for growth on these plays.

“He's anticipating things that are going to happen,” Mariners manager Scott Servais said. “And when you're always thinking ahead like that, your reactions are quicker. Your routes tend to get better. You can kind of see the play happening before it happens. And I think he's one of those players.”

Improving on balls will simply require repetition, Rodríguez said. He regularly shags balls during BP to glean their behavioral patterns off the bat, especially when playing in a new venue for the first time like he was this week in Cincinnati. But realistically, “practice” for screaming line drives in front of him can only realistically be attained during game action.

“You don't know if you're going to have to go back,” said first-base coach Kristopher Negrón, who oversees Seattle’s outfield defense. “Sometimes, you go back and it's top-spinning in front of you, or you go hard in and it just back-spins over your head. So, it's one of the hardest reads and it's tough to perfect.

“But he's done a really good job opening up his hips right off the bat, and as soon as he gets that read, he has that type of speed where he can just get downhill and come in and make the sliding plays in, or when he opens up his hips and sees it deep, he just puts his head down and gets back to the wall.”

Negrón’s analysis can be correlated in Rodríguez’s ability to make the most difficult plays look more seamless than flashy. A review of video data revealed that among Rodríguez’s 307 putouts this year, only one -- yes, one -- have involved him diving, an illustration that his elite range and speed has regularly eliminated the need to dive, and potentially put his body at risk.

According to Baseball Savant, Rodríguez ranks below average in “reaction” off the bat and right at league average in his “route” running. But he makes up for it with his acceleration -- or “burst” -- which is 1.5 feet above league average in the second 1.5-second window off the bat and tied for seventh best. Consider, too, his average sprint speed is 29.7 feet per second, which is well above the MLB average 27.0, and he’s become among MLB’s best in running balls down.

“His closing speed in the gaps has been awesome,” Servais said.

Meanwhile, dissecting his arm strength further is a topic for another day. Last year, he had a 99.6 mph outfield assist to the third baseman, and he’s reached that range in 2023. Overall, Statcast ranks his arm strength in the 90th percentile.

“Everything with preparation, all of that -- having that mindset, having that goal to kind of keep getting better,” Rodríguez said. “I feel like that's what it takes.”

At this time two years ago, Rodríguez was playing center regularly for the first time, at Double-A Arkansas. Then he arrived at Spring Training determined to earn the job despite profiling as a power-hitting corner outfielder. Now, with elite gap-to-gap coverage, two home-run robberies this year and continued improvement on a weakness, he's put together a compelling Gold Glove Award bid.