LONDON - This is a public safety announcement: If you are a ball boy or ball girl at Wimbledon and Milos Raonic is cranking up to serve at almost 150 miles per hour, you'd better take cover.

The giant Canadian cranked up the fastest serve of the tournament on Wednesday with a 147 mph (236.5 km/h) howitzer in a victory over John Millman in the second round.

Raonic also sent down 34 aces, one of which thundered into a ball boy on Court Two while a ball girl also felt the force of another Raonic rocket.

"You don't feel good when that happens," said the 27-year-old Raonic, who was Wimbledon runner-up in 2016.

"Normally by the first reaction of the kid, you can sort of tell how they are, if it hurts, that kind of thing. There was one that hit the boy, but I think he was OK.

"There was one where I hit a girl a little lower in the abdomen. I think she probably took a little bit more of a grunt than he did. I hope she's doing OK."

The ball boys and girls, all drawn from local schools, are no strangers to the power of Raonic.

Five years ago, a ball girl had to leave the court in tears after she was hit on the right arm by a 127 mph (205 km/h) serve during the 6-foot-5, 220-pound Canadian's tie against Carlos Berlocq.

Raonic said it was hard to avoid causing pain－particularly on a grass court, the fastest surface in the sport.

"Everybody is exposed. In that sense it could be a line judge. It could be anything. Because most of the time where the kids stand is if you hit it wide, it's not going to get to them," he said.

"It's more those kind of things if a player guesses the wrong way and it's a serve that's more into the body and the returner just lets it go by, where the kid or the line judge have their guard down.

"That's more where people tend to get hit. Not the serves that are sort of straight through that people are aware pretty early on are going to be aces."

On Tuesday, Nick Kyrgios was clocked at 136 mph (218 km/h) in his win over Denis Istomin.

The Australian also managed to accidentally lay low a ball girl out on Court 12.

"That was tough," Kyrgios said.

"When I heard the sound, I thought it hit the scoreboard. Then I realized it was her arm. It was tough.

"She started crying. She took it like a champ, though. I would have been crying, for sure."

An All England Club spokesperson told AFP there are 250 ball boys and girls－with an average age of 15－on duty at the tournament.

"Training for ball boys and girls begins in February and weekly sessions last 2.5 hours, which put emphasis on match scenario practice," she said.

"There is a strong support network around the courts consisting of ball boy and girl assistants and senior instructors, in addition to there being appropriate medical care."