Heat illness was apparently the reason for the top seeded Romanian's retirement shortly after she lost the second set of the quarter-final at Rock Creek Park Tennis Center.
The Russian's 2-6 6-3 1-0 success means she will face Oceane Dodin for a place in the final, after the Frenchwoman's 5-7 6-4 7-6 (7/4) defeat of Germany's Sabine Lisicki.
Murray will not be 'wading in' with advice for Raducanu
The other semi-final is an all-German affair.
Julia Goerges beat Romania's Monica Niculescu 6-3 6-4 to set up a match against Andrea Petkovic, who beat Canada's Bianca Andreescu 6-7 (3/7) 6-1 6-2.
Results from the Washington Open Women's Singles Quarter-final matches on Friday
- 4-Julia Goerges (Germany) beat 6-Monica Niculescu (Romania) 6-3 6-4
- 7-Ekaterina Makarova (Russia) beat 1-Simona Halep (Romania) 2-6 6-3 1-0 (Halep retired)
- Andrea Petkovic (Germany) beat Bianca Andreescu (Canada) 6-7(3) 6-1 6-2
- 5-Oceane Dodin (France) beat Sabine Lisicki (Germany) 5-7 6-4 7-6(4)
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