Napoli stung Tanaka by hitting a solo home run with two outs in the ninth inning, lifting Jon Lester and the Red Sox over the Yankees 2-1 on Saturday night.
"What an idiot!" Napoli was heard exclaiming on Fox television microphones as he high-fived teammates in the dugout.
Napoli later said he didn't mean any disrespect, adding he was "surprised" Tanaka didn't throw his tremendous split-finger fastball.
Napoli had struck out in his previous two at-bats, was down 1-2 in the count this time and was having all sorts of trouble with Tanaka's diving splitter.
"He had me where he wanted me," Napoli said.
Tanaka, however, preferred to throw a fastball. Napoli lined it the opposite way, into the first row of seats in right field.
Napoli, who also homered off Tanaka at Fenway Park in late April, raised his right arm as he rounded first base and clapped his hands as he headed for second.
Tanaka, the top winner in the majors and the American League ERA leader, turned to watch the ball sail, twisting his body when it cleared the wall.
"It was the worst thing I could've possibly done," Tanaka said through a translator.
Tanaka wanted to take a different approach than catcher Brian McCann.
"He asked for a splitter and for a slider, and I shook off both of them," he said.
Tanaka said he intended to throw a fastball out of the strike zone to set up a breaking ball.
Instead, Napoli hit it out of the park. His third home run in five games, and 10th shot overall, flew far enough to reach the short porch.
"Power hitters are going to have the ability to hit to all fields. We all know that right field here is not very forgiving," Red Sox manager John Farrell said. "Last night, they hit one in the first row. Tonight, we did."
The Red Sox won for just the third time in nine games. The victory made the defending World Series champions 37-44 at the midpoint of the season -- it's the first time since 1997 that Boston has been under .500 at the halfway mark.
Lester (9-7) held the Yankees hitless until the sixth. He gave up an unearned run and five hits in eight innings, striking out six and walking two.
Koji Uehara pitched a perfect ninth for his 17th save in 18 chances.
Tanaka (11-3) allowed seven hits in a complete game, striking out eight and walking one.
The Yankees lost for the fifth time in seven games. Tanaka dropped consecutive starts for the first time in the majors.
The matchup between Lester and Tanaka shaped up as a pitchers' duel, and it certainly was. Lester improved to 13-6 lifetime against the Yankees, including a loss to Tanaka in Boston on April 22.
Lester and Uehara came through for a Red Sox team that has scored three runs or fewer in 12 of its past 14 games.
Napoli did his part, too, tagging Tanaka.
"He pitched pretty well to Nap all night," Lester said. "I'm guessing Nap put a pretty good swing on that ball right there. Luckily, we're in Yankee Stadium and not anywhere else and that ball goes out."
An odd sequence ended the Yankees' eighth. Jacoby Ellsbury tried to steal second with two outs, and the fans cheered when catcher David Ross' throw skipped into center field. As Ellsbury headed toward third, however, the crowd began to realize strike three had already been called on Mark Teixeira.
Ross homered in his second straight game, launching a drive far over the left-field fence in the third. Tanaka muttered to himself as Ross rounded the bases.
Lester, who threw a no-hitter against Kansas City in 2008, held the Yankees hitless until Brett Gardner bounced a leadoff single up the middle in the sixth.
Lester's bid was extended with two outs in the fifth when Yangervis Solarte was called out on a video review, taking away an infield single.
Pedroia opened the next inning with a single and tried to test the arm of Ellsbury, his former teammate. The Yankees center fielder made an accurate throw and Pedroia was called safe, but he was ruled out after New York challenged the umpire's decision.
NOTES: Of Boston's past 14 games, 10 have been decided by one run. The Red Sox are 6-4 in those games. ... The Red Sox are 26-26 at the new Yankee Stadium. ... Boston OF-INF Mike Carp (right foot) has started his rehab assignment at Triple-A Pawtucket. ... Yankees LHP CC Sabathia (right knee) made his first rehab start, pitching 2S innings for Class A Tampa and giving up two runs on three hits and a walk versus Dunedin.