The giant Russain withdrew from his quarter-final with Korean qualifier Lee Hyung-taik in the hope of recovering in time for the first grand slam of the year.
"I need to stop for a few days... but I hope I will be okay," Safin said.
"(The doctor) said it's quite serious and it was better to stop right now."
Safin's withdrawal provided Lee with a free-ride into Friday's semi-finals where he will meet either Franco Squillari or Wayne Ferreira, who play in Thursday's last quarter-final.
Ferrero is the only seeded player left in the men's draw after he blasted aside Thailand's Paradorn Srichaphan 6-3, 6-1 in brilliant fashion.
The Spaniard will play Rainer Schuettler for a place in Saturday's final after the German beat Mardy Fish 7-6, 6-4.
While the men's draw has been decimated by big-name casualties, the women's tournament has thrown up an intriguing semi-final showdown between Kim Clijsters and Justine Henin-Hardenne.
Clijsters crushed Chanda Rubin 6-1, 6-2 while newlywed Henin-Hardenne had a tougher workout from South African Amanda Coetzer before triumphing 6-4, 6-4 to set up an all-Belgian semi-final.
"Kim and me, we are both fighters and we want to prove that Belgium is a small country but you can make great things in a small country," Henin-Hardenne said.
Russian giantkiller Tatiana Panova, who knocked out top seed Jennifer Capriati in the first round, booked her place in the semi-finals with a comfortable 6-3, 6-2 win over Olga Barabanschikova.
She will play either Lindsay Davenport or Daniela Hantuchova in the semi-finals.
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