Graf and Agassi were holding hands — her left to his right — while rallying with a couple of youngsters when Agassi's follow-through struck his wife in the face.

Agassi held his racket in his left hand.

After she was hit, Graf lifted her left hand to her mouth and walked off to the side of the court with Agassi following closely behind her to check on her well-being.

She wiped her mouth with a towel before leaving the stadium for an on-site doctor to administer three stitches to her lip, officials said. It was not clear whether the cut was to the inside or outside of her mouth area.

She suffered no damage to her teeth, officials said.

"She's OK," Agassi said. "It was an unfortunate accident."

Agassi and Graf were in Houston because Agassi is part of a reality show called 'The Big Give,' an upcoming Oprah Winfrey production. An auction benefiting an elementary school in the Houston vicinity chosen for the show raised at least $225,000 Sunday.

The doctor who paid $70,000 for a trip to play tennis with the couple stitched up the multiple Grand Slam winner.