'What was that?' a spectator asked, but the enquiry could have come from the bewildered young German facing the Wimbledon champion, whose first service game yielded three aces. The answer is the latest puzzle concerning a 23-year-old man already covered in several thick layers of enigma.
Agassi, ever unconventional, had not surprised with his clothes or his manner this time. Instead he had unveiled a completely novel stroke - more a swat of at an imaginary fly than a full- blooded blow. Could a man alter an integral component of his game sufficiently to stand the test of a Grand Slam championship in so short a time? Probably no-one in the world other than Agassi.
The rumours have been circulating the All England Club this week that Agassi had spent part of his lay-off since April at Newport, Rhode Island, trimming the borders of his grass-court game. Nick Bollettieri, his coach, is reported to have said the serve was the results of six weeks' work, but the man himself is insistent in his denials. He claims he arrived in Germany 12 days ago with his practice time curtailed to just 45 minutes by tendinitis of his right wrist.
The new serve was his own idea. 'The full back-swing hurt me,' he said, 'so I cut the motion down. I'm 50 per cent comfortable with it and 50 per cent tentative.' His opponents so far have been less comfortable, as he has already served 26 aces, compared to 35 in the whole tournament last year.
There is considerable pace generated too by a motion that the American press have christened the 'tomahawk chop'. IBM, who monitor service speed, have clocked Agassi at a maximum of 118mph, the ninth fastest serve in the championship. Last year, in seven matches, the fastest Agassi could manage was 110mph. 'I can't change it now,' he said. 'I'm committed to it, win or lose. I feel it's really won me some key points and I feel I'm moving the ball round real well. So I have no problem keeping it.
'My wrist is fine, I'm hitting the ball as hard as I want. If I have had a struggle, it's hitting it too hard. I'm just trying to zero in and get my game sharp right now. So power is not a problem.'
Agassi's presence in the fourth round and the second week at Wimbledon has confounded expectation and delighted those who crave characters in a sport desperately short of them. He has dominated the build-up and the first week of Wimbledon and he will continue to be the central figure as long as he survives. That the championships prospered without him for three years from 1988 to 1990 now seems remarkable.
He feared his appearance as the defending champion might be short, but so far he has won his matches easily without needing his finest form. Better, more seasoned, grass-court players than Karbacher or Joao Cunha-Silva (his second-round opponent) would not have let go of the jugular when Agassi was in trouble but he has ridden his luck and must be considered a genuine contender.
'The key to the tournament for me was the first few matches,' he said. 'They were going to be the toughest. Now I think there's a realistic shot of winning the tournament. I'm feeling good about my tennis, it's go out there and see what kind of shots I can come up with at the right time. I'm pleased with how things have turned out.'
Like most baseliners, he dislikes shots sliding through on soft, wet surfaces but this time the fine weather and compacted turf has played into his hands. Martina Navratilova, who knows Wimbledon as well as anyone, believes the ball is bouncing higher at an earlier stage of the tournament than ever before. 'I was swinging returns round my waist the first couple of days,' Agassi said, 'and now they're getting up round my shoulders now and then. I remember last year by the final the ball was bouncing up almost every shot. It's the height of the bounce that has a lot to do with the ability to manouevre the ball around.
'That's my zone where I generate a lot of power. It's the same reason why (Jim) Courier and other baseliners like the ball sitting up. It gives us a chance to really use what we're good at: which is our angles, which is our put-away, which is our top-spin lobs. When the ball's skidding low it takes away all out options and the serve and volleyer has the advantage.'
It is a big server he will face tomorrow. Richard Krajicek is the ninth-seeded Dutchman who has power in abundance and is typical, in Agassi's opinion, of the breed you meet in the latter rounds at Wimbledon. The 21-year-old may lack subtlety but he forcibly ejects opponents off court.
Last year Agassi dealt with might by taking Goran Ivanisevic's serve on the rise and using the power against its source. Whether his reflexes are match sharpened enough to control his racket head this time is debatable.
Certainly Krajicek is a place ahead of the American in the rankings and has an 11-5 win-loss record while Agassi has been idle. In the last Grand Slam tournament, the French Open last month, Krajicek reached the semi-finals on a clay surface which is not his best.
The omens are not good for Agassi, although the Las Vegan did not read the form lines last year and does not seem inclined to regard them now. 'It's just making sure I'm disciplined mentally to work and work and work to get your chances. That's what I had to do last year and I came here knowing the same thing. I think I'm ready.'