Williams has said the WTA is too harsh with its rules for injured players
Serena Williams quit her first-round Madrid Open match against Francesca Schiavone saying she had to protect an injured knee - and her finances.
Williams, who pulled out after losing the first set 6-4, has been critical of the WTA Tour's policy on injuries.
"I don't really have a choice whether to play," said Williams, who has said players would be fined for not playing even if they were dead.
She added: "I'm not into throwing away thousands and thousands of dollars."
The world number two - second seed in Madrid - said she was currently remodelling a house and said: "I don't know about anyone else but $75,000 is a lot of money to me.
"That's like my whole furniture bill, and some stairs, rugs."
The 27-year-old won her 10th Grand Slam at the Australian Open in January.
However, she insisted her injury would not affect her chances of playing in the French Open later in May.
"I didn't want to risk my chances to play Roland Garros," she said.
"I wanted to do well [here], but at the end of the day I'm trying to play Paris and still trying to play singles and doubles there so I didn't really want to take that chance and be out of Roland Garros."
In the second round on Monday, Russian third seed Elena Dementieva cruised past Lourdes Dominguez Lino, beating the Spaniard 6-3 6-2.
Her eighth-seeded compatriot Nadia Petrova was made to work hard by Germany's Anna-Lena Groenefeld before triumphing 4-6 7-6 (7-3) 7-6 (7-5).
Switzerland's Patty Schnyder eased to a 6-2 6-4 win over Aleksandra Wozniak of Canada to also reach round three, while Elena Vesnina of Russia beat France's Virginie Razzano 7-5 6-3.
In Monday's first-round matches, Slovakia's Daniela Hantuchova beat Gisela Dulko of Argentina 7-6 (9-7) 6-1.
And France's former world number one Amelie Mauresmo saw off Ai Sugiyama of Japan 6-2 6-4.
There were also wins for Varvara Lepchenko of the USA, China's Peng Shuai and Italian Roberta Vinci.