CHICAGO -- When he first came up to the big leagues with the Yankees, White Sox closer David Robertson looked down to the back end of the bullpen and saw an iconic pitcher in Mariano Rivera with an iconic pitch, his cutter.
Now an established reliever and likely one of the hottest closer targets available at the non-waiver Trade Deadline, Robertson has his own signature pitch.
The 32-year-old has thrown his curveball 40 percent of the time this year, a career high, and has done so with astonishing success, holding hitters to a .091 batting average against it.
It's no secret that Robertson's value remains high as the trade rumors come swirling; he has posted a 2.29 ERA through his first 18 appearances while converting eight of his nine save opportunities this season. He has been a reliable figure in a surprisingly good Chicago bullpen, whose 2.46 ERA ranks second-best in the Majors despite injuries to Nate Jones, Jake Petricka and Zach Putnam.
"[David] actually looks very, very calm out there right now, very poised," White Sox manager Rick Renteria said. "There's a reason why he's able to effectively go through a lineup in those moments and be able to close out a ballgame. It doesn't always happen, but for the most part you feel very comfortable when he's out there on the hill and giving you what he's got."
Part of Renteria's comfort comes in Robertson's curveball, which has emerged as one of the single nastiest pitches in the game.
According to Statcast™, Robertson has the highest swinging-strike rate -- the number of swinging strikes per pitch thrown -- among curveballs in the Majors at 28.23 percent. His curveball also has the highest whiff rate (number of whiffs per swing) in the Majors, getting hitters to miss 59.32 percent of the time.
Robertson, who most often employs a cutter that bears in on lefties to counter his breaking ball, said his comfort with the curveball mostly relies on his ability to throw it no matter what the situation may traditionally suggest he should throw.
"Throwing a few for strikes helps," Robertson said of the pitch. "Guys, when they come into the box, they know that I'll throw it at any point, any time for a strike. It gets in the back of their heads that they have to be on alert and put the ball in play because I'm trying to throw strikes and get guys out as quickly as possible."
Robertson always has that elite pitch in his back pocket. For example, in his appearance against the Red Sox on Monday, he earned his third save in as many days by offering a steady diet of cutters and fastballs to the first two hitters, inducing a strikeout and flyout in the process. It wasn't until pinch-hitter Sandy Leon came to bat with two outs that he even pulled out the curveball.
He threw it four times in the six-pitch at-bat, including the final pitch of the game, a breaking ball buried in the dirt for a strikeout.
"I think his breaking ball's really sharp right now, and his fastball command is really good," Renteria said. "You have a combination of those two things, and it's pretty effective."
It's a learning curve that hitters have yet to figure out.
Fabian Ardaya is a reporter for MLB.com based in Chicago. This story was not subject to the approval of Major League Baseball or its clubs.