London - Aston Villa's Jack Grealish was punched by a pitch-invading Birmingham fan in a shocking incident during Sunday's Championship clash, but the midfielder had the last laugh as he scored the decisive goal in a 1-0 win over their bitter rivals.
Grealish was in the Birmingham penalty area in the 10th minute when a man ran onto the field and threw a punch from behind at the Villa captain.
The blow caught Grealish on the side of the head and sent him sprawling to the turf.
The intruder was quickly restrained by a steward as Villa players including Tammy Abraham and Glenn Whelan rushed to Grealish's defence.
After the fan had been shoved over, he was eventually led away by police, defiantly blowing kisses to the crowd.
A visibly furious Grealish was helped to his feet and was able to continue playing once order was restored at Birmingham's St Andrew's stadium.
Grealish produced the perfect response to the assault in the 70th minute.
Taking possession 20 yards from goal, he advanced into the Birmingham penalty area before drilling a low shot into the far corner to seal Villa's league double over their hated neighbours.
In a fittingly dramatic gesture, Grealish jumped into the stand to celebrate with the Villa fans behind the goal and was booked as a result.
"I was walking into position and then just felt a whack around the side of the face," Grealish said.
"Obviously there's rivalry and stuff in football but I don't think there's any place for that really.
"I just tried to get on with my job. To score the winner was unbelievable, after what happened in the first half I think it was set up for it.
"Best day of my life. To come here, captain Aston Villa and score the winner is what dreams are made of, especially for me as a Villa fan."
West Midlands Police confirmed the Birmingham supporter had been arrested, while the Blues quickly apologised for the incident.
"We deplore the behaviour of the individual who committed this act and rest assured he will be banned from St. Andrew's for life," a Birmingham statement said.
"What happened has no place in football or society. Jack is a Birmingham lad and regardless of club allegiance should not have been subjected to this - there are no excuses."
Villa boss Dean Smith added: "It shouldn't happen. Hopefully the lad gets locked up and luckily Jack Grealish is okay.
"Security's got to be ready for people coming on to the pitch but then some lunatic goes on trying to make a name for himself."
There is a long history of animosity between Birmingham and Villa supporters, with outbreaks of serious violence in the streets marring many of the 'second city' derby encounters.
There was fighting between rival gangs before Sunday's match, with police having to hold back fans while at least one man was seen being treated by paramedics after suffering a head wound.
Fanning the flames further, Grealish is a boyhood Villa fan and once turned down a chance to join Birmingham on loan when he was a youngster, reportedly saying: "I'm not going there".
In 2002, a Birmingham fan was jailed for four months after running onto the pitch at St Andrew's to taunt then Villa goalkeeper Peter Enckelman, who had just conceded a bizarre own goal when he let a backpass roll under his foot into the net.
Adding to the tense nature of Sunday's derby, both teams are fighting to earn a place in the Championship play-offs, with Villa's win taking them one point above Birmingham.