In a game often won by the km/h of an aced serve or the millimeters that mark a ball's errant path, greatness can be measured by an intangible called guts. Germany's Steffi Graf, the reigning queen of tennis, has guts in abundance. It was inner fortitude that helped her come back earlier this month to win the longest women's final in French Open history. It takes mental toughness for Graf to win big matches, especially when her father is sitting in jail and tax investigators are trying to account for every bit of money she ever made.

This week Graf returns to the hallowed grass courts of Wimbledon, where she strides with the ease and confidence befitting a six-time winner and defending champion. But the pressure will be even more intense than usual, for history is in the making. If she wins--and London bookmakers are offering 4-7 odds that she will--Graf will capture her 100th career title as well as her 20th Grand Slam tournament victory. That would move her past American Helen Wills Moody and leave her trailing only Australia's Margaret Smith Court, who holds 24 major titles, on the all-time list of Grand Slammers. Graf has already passed contemporaries Chris Evert and Martina Navratilova. Eventually catching Court is not an impossible feat for Graf, considering that she is just 27 years old.

Not that Graf's place in tennis history as one of the greatest, if not the greatest, could ever be in doubt. While archrival Monica Seles still struggles in her comeback from being stabbed three years ago by a deranged Graf fan, Steffi totally dominates the sport. She has won the past four Grand Slams she entered. Now in her record 338th week as the world's No. 1 player, she has been remarkably consistent despite numerous injuries. "If you punched into a computer" all the attributes you wanted in a female athlete, says former Wimbledon champion Virginia Wade, "the result would be Steffi. She's amazing." Two-time U.S. Open winner Tracy Austin puts her admiration in even plainer terms: "If I had to pick one person to play a singles match with my life on the line, I'd pick Steffi Graf."

The courts that give her trouble are not grass or clay but civil and criminal. Her father Peter, 58, was arrested in Germany in August 1995--just before Graf's fourth U.S. Open victory--on suspicion of hiding $28 million of her tournament and endorsement income in accounts in the Netherlands, the Netherlands Antilles and Liechtenstein. He will probably go to trial in September on charges of tax evasion, and he could face up to 10 years in prison and payment of $12 million in back taxes. Although she has not been indicted, Graf remains under investigation despite her repeated denials that she played any role in or had any knowledge of the management of her finances.

Rich Steffi, Poor Child, a book just published by the German newsweekly Der Spiegel, presents evidence that she played a bigger role in the tax dodging than she would have everyone believe. In 1990, the book claims, the Grafs' tax adviser, Horst Schmitt, called a meeting with the whole family and spelled out in detail the anticipated tax problems. A source described as "a close friend of the family who was told by a member of the clan" insists that Graf was present. "I don't remember such a meeting," she told TIME. "But I don't want to get into it."

Soon she may have to. Even as she is negotiating with the prosecutors and tax authorities--a one-time payment is possible if they decide she did not know anything--the paper trail seems to be leading right to her door. According to Der Spiegel, investigators have uncovered contracts bearing Graf's real signature, not a machine-stamped one, and other documents that indicate she knew what was going on.

In view of the chaos in her personal life, it is remarkable that Graf keeps winning. At both the 1995 U.S. Open and the French Open earlier this month, she broke down in tears when the subject of her jailed father came up--but not until she had won the titles. "Something like this probably makes her even more focused, more determined, more concentrated on what she wants to do," says Austin. "She seems to put all her energy into the tennis and tries to focus on that so that she doesn't have to think about all of those off-court problems...It's kind of a haven for her out there."

But closing herself off comes at a price. "Steffi is a really quiet and shy person [who] likes to be around the people that care about her," says Rennae Stubbs, one of her few close friends on the women's tour. "Being who she is, she has to be very wary and aware of people and who she has around her." Not so long ago, the most important of those people was her father, who was her mentor, coach and caregiver. Having that bond broken has made her inner circle--which includes her mother and brother as well as coach Heinz Gunthardt and boyfriend Michael Bartels, a German race-car driver--draw even closer.

Still, the famously reticent Graf allows that life isn't a solo sport that can be won by guts alone. "There have been a lot of times when I've been really down these past months," she says, "and somebody comes up to me and wishes me well. That makes a difference."

--With reporting by Kate Noble/Eastbourne and Bruce van Voorst/Bonn