Hillary Clinton. AP Donald Trump's strongest moment suddenly turned into one of his weakest.

Hillary Clinton was on the defensive at the third and final presidential debate. The candidates were talking about immigration. One of her paid speeches to a Brazilian bank, a transcript of which WikiLeaks released as part of hacked Clinton campaign emails, was under the spotlight.

Then she changed the subject.

She veered off the discussion of "open borders," which the transcripts showed her praising in her speech, and instead started talking about Russian President Vladimir Putin. She accused Putin and the Russian government of meddling in the US election, and she called on Trump to disavow "Russian espionage against Americans."

Trump responded with a zinger, marveling at how Clinton had shifted gears. And then, for some reason, he took the bait.

"Now we can talk about Putin," he started.

It was a seminal, if unheralded, moment as part of Clinton's master class in debate sparring with an unworthy partner. In romping through the nominees' three debates — victorious in each and opening up a more and more sizable poll lead — she was the class president versus the class clown, the well-prepared interviewee versus the guy winging it on the biggest stage of his life.

Clinton did what 16 Republican candidates couldn't in the GOP primary: beat Trump at his own game.

She trolled him.

On Wednesday, that came when she relentlessly needled him over his meeting with the Mexican president, which led to a dispute over whether he had actually discussed building a Southern border wall and who would pay for it.

"He choked and then got into a Twitter war," she said.

She mocked him.

There was the exchange Wednesday during which she grinned and interjected as he was talking about sitting in an apartment in his "very beautiful hotel" in Las Vegas.

"Made with Chinese steel," she quipped.

She outplayed and outprepared him.

In the first debate, it was invoking Alicia Machado, the former Miss Universe winner with whom Trump engaged in a weeklong feud. This time, Trump was so obsessed with defending himself against allegations of sexual misconduct from several women that he allowed her to avoid a question about former President Bill Clinton's own questionable history with women.

In slicing through his opponents in the GOP primary one by one, Trump often made other candidates the foil on the debate stage. His most frequent target was Jeb Bush, whom he routinely made a punch line.

On Wednesday night, at the end of three general-election debates, Trump was his own punch line.

"Nobody has more respect for women than I do. Nobody," he declared.

And the audience laughed.