"Harder than Brad's," a smiling Agassi replied, with reference, if not deference, to his coach, Brad Gilbert, who once won $1m (pounds 600,000) as the runner-up at the Grand Slam Cup in Munich. "I assure you, it's bigger than Brad's."
Agassi and Graf had been hitting on a secluded court when the pair were asked to transfer to the main arena as a favour to the Eurocard Open tournament director, Markus Gunthardt, who was keen to entertain spectators during an unscheduled gap in the programme. Agassi had been due to play doubles with Nicolas Kiefer, the German No 1, who retired from the tournament after damaging ankle tendons during his second round match against Andrei Pavel, of Romania.
Gunthardt, the brother of Graf's former coach, Heinz Gunthardt, may even have twigged to the notion of having a public mixed practice session at one of the world's premier men's tournaments after receiving a letter from Serena Williams, the 18-year-old United States Open women's singles champion, dated 10 October.
It would be of extreme delight if you would grant me - Serena - a wild card into your tournament, the `Eurocard Open', in Stuttgart, Germany.
During the tournament I will be attending school, but will quickly leave to be a part of the tournament.
If a wild card in singles is not available, I would love also to have one in the doubles - preferably with Pete Sampras, or whoever else desires to play by my side. I am sure they will win with me!
Thanks so much for the wild card in advance, I am so happy!
Serena's tongue-in-cheek request was refused because of an ATP Tour technicality: theirs is a men's tour, and she is not a man.
The Graf-Agassi interlude was significant - "symbolic," one German journalist suggested - because about 65 per-cent of the German press coverage next day was largely devoted to the cameo, even though Kiefer was a wounded German warrior fighting for a place in the ATP Tour Championship in Hanover next month.
But, then, Kiefer, though highly gifted, is not Boris Becker, or even Michael Stich. Likewise, Tommy Haas, the German No 2. Nor do Kiefer and Haas even dream of approaching the status of Steffi Graf.
Moreover, there is nothing like a touch of romance. Graf and Agassi have been spending time together at every opportunity since the newly- retired, seven-times Wimbledon champion was seen standing among the spectators at the US Open, cheering Agassi to victory. They went to the Felix Trinidad v Oscar De la Hoya fight in Las Vegas, and stayed on in Agassi's home town for his annual charity gala at the MGM Grand.
They then travelled to Germany. Agassi participated in the Grand Slam Cup in Munich; Graf visited Vienna to see Willy Dungl, the specialist who helped her through her ailments last year.
Apart from receiving various awards, Graf has been to South Africa, where she sponsors a charity in the townships, "Children for Tomorrow". Afterwards she went to meet Agassi in San Francisco. They arrived in Stuttgart together, and Graf watched all Agassi's matches last week, usually seated beside Brad Gilbert; sometimes smiling, sometimes pensive.
Agassi is accustomed to high-profile relationships. A few years ago he was to be seen in Barbra Streisand's company (Streisand once described him as a "Zen Master"). Agassi's two-year marriage to the actress Brooke Shields ended in divorce in April this year.
Graf, by comparison, has tended to be conservative in her choice of boyfriends: Alexander Mronz, a German tennis player, and Michael Bartels, a German racing driver. Her friendship with Agassi appears to have been greeted with surprise and delight by the public, and with a degree of scepticism by some tennis insiders.
"Andre and I had a wonderful time in America," Graf told Stern magazine, "but the relationship is very young and I want to keep a little area of my life private. We heard in Las Vegas there had been some wild stories and speculation about us in Germany, but we were not secretive. Not even in my dreams would I have imagined the amount of fuss there has been, and the number of paparazzi."
The sight of Agassi and Graf on the court raised speculation that they may be planning to play mixed doubles at the Australian Open in January. "No," Agassi said. "Got to separate business and pleasure."
Does playing in front of Graf make a difference? "No. I mean, I'm out there working. She's one of, in this case, probably 9,000 people there. She can't change the way I hit the ball, that's for sure. I wish she could. If I had her slice today, maybe that would have neutralised Enqvist a little bit." Thomas Enqvist, of Sweden, defeated Agassi in the semi-finals, 6-3, 4-6, 6-0.