As James Blake crossed the finish line Sunday, Victoria Azarenka, a fellow tennis pro, was there to make good on a Twitter promise struck Saturday: a chocolate milkshake from Shake Shack.

Blake needed it. He said the course was difficult. He is a retired tennis pro, once ranked the No. 4 player in the world; but he is not a professional runner.

"Halfway through, I wasn't sure it was possible," Blake said at the finish line at Central Park, having clocked in at 3 hours, 51 minutes and 19 seconds. "I didn't think I'd be able to get through four hours."

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The first-time marathoner, who ran to support cancer research on behalf of the James Blake Foundation, is also juggling a national role spotlighting police brutality.

In September, Blake was at a Manhattan hotel when a plainclothes NYPD officer tackled him after mistaking him for a suspect in a credit card fraud scheme. The arrest, captured on surveillance video, drew public outrage over the police officer's tactics. Mayor Bill de Blasio and Police Commissioner William Bratton apologized to Blake.

Blake, who retired from tennis after the 2013 U.S. Open, made no mention Sunday of his campaign to end police brutality. He said he felt he had the crowd on his side. They called to him: "Go, James" and "Go, Blake."

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"It makes you not want to stop," he said, clearing his throat. "A lot of people told me that, but I didn't believe it."

Blake said he is used to support from the crowd. But this was not a tennis match -- where he was "good" at the sport.

Instead, it was "very cool that perfect strangers are cheering for you in something you're not that good at," Blake said.

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His voice was hoarse.

"My voice sounds really weird or is it just me?" he asked a reporter.

Will he run again?

Blake, 35, shook his head several times.

"Leave it to the pros," he said. "Leave it to the people who can train a little more and have this kind of energy. I don't know if I'll have this kind of energy again."