Friday, 11 August 2017: When Peter Moylan first heard the crowd cheering he thought something funny was being played on the scoreboard.

The cat was grey, not black, but it was still bad luck for the Kansas City Royals' Australian pitcher Peter Moylan.

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The Royals were up 5-4 in the sixth inning against Major League Baseball rivals the St Louis Cardinals on Wednesday with Moylan on the mound.

The Australian was feeling good and needed just one more out to end the inning when the small, fluffy cat appeared out of the blue and ran across St Louis' Busch Stadium.

The crowd cheered.

A groundsman ran on, chased after the cat, grabbed it and then suffered as the feline scratched and bit him.

The crowd cheered each time the grounds man grimaced in pain.

The players laughed.

After the two-minute intrusion Cardinals' batter Yadier Molina stepped up to the plate, the bases were loaded and Moylan threw a pitch that was supposed to be low and move away from Molina.

Instead it sat up in Molina's hitting zone.

He smashed it into the left field seats for a grand slam home run in a game-changing play that led to the Cardinals' 8-5 victory.

"I'm not a cat person but I sure liked that one," Cardinals manager Mike Matheny told reporters.

Moylan, a 38-year-old from Western Australia who has also had stints at the Los Angeles Dodgers and Atlanta Braves, was not so complimentary about the intrusive cat.

"I thought it must have been something with the scoreboard," Moylan said.

"I turned around and there's a kitten running around, which is the first time that's happened to me."