If ball kids were paid, they might get danger money when American John Isner is playing tennis.

At the least, his matches should come with a warning because, at 2.08m tall, Isner can hit a big serve.

He hit ball girl Beatrice Smit, 13, in the shoulder with a 231km/h bullet yesterday in his 7-6 (1), 3-6, 6-2 win over Slovakian qualifier Lukas Lacko.

Isner immediately winced, fearing he had injured her, but he needn't have worried. "I went up to her and said, 'I'm sorry,"' he explained. "And she said, 'No, it was a good serve.'


"Sometimes you play and I'm worried about people in the crowd because I can get the ball up. I have hit people in the eye. It's just terrible. Luckily for me, she was tough as nails out there. She didn't flinch. She dealt with it fine but that was a huge serve.

"I hit a person in the eye in Washington DC a few years back. That was the worst. That was ugly.

"I don't think I have seriously hurt anyone ... yet. The girl took one here. Six inches [15cm] higher, I could have got her in the face," the American said.

A large part of Isner's game is based around his serve and last year the world No14 served the most aces on the ATP Tour (979) and he also holds the record for the most aces in a match (113).

His fastest recorded serve is 241.2km/h in 2011, which places him 11th on the all-time fastest list. Australian Samuel Groth holds the record for the fastest recorded serve with 263km/h set at a Challenger event in South Korea in 2012.

What was significant about the serve that hit Smit was that it was out wide - normally the fastest are set when players serve down the middle where the net is lower. It happened soon after a disputed line call and as he closed in on the end of the match.

"It's the adrenalin, whether I'm down break point in a crucial point of the match or I'm up a break and serving for the match," Isner said when asked when he tends to serve his fastest. "A lot of times I have served for the match and hit four aces before because I have the adrenalin."

That fire also helped him overcome an ankle injury that saw him pull out of last week's Hopman Cup. He flinched a couple of times during the match with Lacko but battled on and was hopeful it wouldn't flare up again before his quarter-final showdown with fifth seed and world No22 Philipp Kohlschreiber.

The Heineken Open is important to Isner, but not if it comes at the expense of his chances at next week's Australian Open.

"[The ankle is] very tender and it's wrapped very tightly," he said. "It's going to be interesting to see how it turns up in the morning. I think I will be fine. It will have to be hurting really bad for me not to take the court."

Kohlschreiber has a good record against Isner, having won their last two matches, and an even better one in Auckland where he has been a quarter-finalist or better in nine of his 10 visits to the Open. He was a beaten finalist last year and won in 2008 and Isner described him as someone who plays "smart" tennis against him.

It looms as an intriguing match - but ball kids need to be alert.