The top seed and world No 3 was trailing 3-0 in the third set, having won the first 6-3 and lost the second 5-7. Pierce had pulled out of the doubles earlier in the week after suffering from a kidney infection.
Begerow took early control of the second set, breaking in the second game and holding serve to love. Pierce, looking increasingly slow and erratic, left the court for several minutes at 1-4, but the break failed to slow Begerow's momentum.
Having lost the second set, Pierce barely competed in the third, often failing to make an effort even for the easiest ball, and it was no surprise when she retired. She slumped into her chair, failing even to shake the winner's hand at the net.
"I've had a stomach problem that the doctors said might be a minor kidney complaint, and I've been taking antibiotics," said Pierce. "I've been practising for several days without any problems, but I guess I wasn't ready to play a match."
Her withdrawal weakens the top half of the draw and boosts the hopes of reaching the final of the Swiss teenager, Martina Hingis, who meets Judith Wiesner in the quarter-finals.Reuse content