OAKLAND -- Given his vantage point down the left-field line, fellow A's reliever Andrew Bailey isn't sure what hitters see in Michael Wuertz's slider.

"Sitting in the bullpen," Bailey said, "we see it bounce two feet in front of hitters, and we're like, 'Why are they swinging at that?'"

Maybe that's what makes it so good.

Wuertz's slider has been the toughest pitch to hit in all of baseball this season.

The pitch has a 49.7 percent swing-and-miss rate, narrowly ahead of Royals starter Zack Greinke's slider (49.0 percent) for tops in the Major Leagues, according to baseball analyst Harry Pavlidis in a July 15 report in the Wall Street Journal.

The Cubs' Rich Harden's changeup (48.6 percent), the Yankees' A.J. Burnett's curveball (48.3 percent) and the Rockies' Jorge De La Rosa's slider (42.5 percent) round out the top five.

"We call it the 'drop ball,'" Bailey said. "It seems like it's impossible for hitters to make contact against it, and when they do, it's bad contact. It's basically unhittable."

It's been plain unhittable as of late. Wuertz recorded a season-high four strikeouts on Friday -- all on sliders, of course -- giving him 27 punchouts in 13 2/3 innings over his past 10 appearances.

Wuertz ranks second among American League relievers with 57 strikeouts in 44 1/3 innings. He's 5-1 with a 2.84 ERA in 42 appearances this season.

Oakland manager Bob Geren called the pitch sharp and late-breaking, just like a normal slider. He said Wuertz's three-quarter arm slot is a big reason the pitch has been so successful.

"It's deceptive," Geren said. "It gives him better rotation than most. ... It looks like a fastball until it's time for the hitter to make a decision to swing or not."

Then the hitter swings and misses. Strike three. Over and over again.