Former world number one Venus Williams battled past fellow American Laura Granville 7-6, 6-4 to reach the Memphis International semi-finals on Thursday.
The seventh seed, continuing her comeback after more than four months out through injury, won the first set in a tiebreak and came from 3-0 down in the second set to clinch victory.
Williams will meet Romanian qualifier Ioana Raluca Olaru in the semi-finals after the 16-year-old advanced when fourth-seeded Australian Samantha Stosur retired at 5-1 down due to a virus.
American Williams stormed into a 4-1 lead but world number 66 Granville battled back well to force a tiebreak, only for the seventh seed to take it 7-2.
A third set looked a real possibility when Granville broke on her way to a 3-0 lead in the second but Williams won five straight games and held three match points on her opponent's serve at 5-3.
Against the odds, Granville held serve but the respite was only brief as Williams served out to love, clinching victory with her seventh ace.
"There were games where I was making four errors," Williams told reporters.
"A lot of times I just wanted to hit such a great shot that I ended up hitting not such a great shot at all. So I have to hit shots a little more average and set myself up a little better.
"But I feel like I played a really good match, especially from the 0-3 point I felt like I started getting my feet in a good position. And my serve has definitely improved since the first match."
Top seed Shahar Peer cruised into semi-finals with a 6-0, 6-3 win over American Bethanie Mattek.
Peer, who did not drop a game in the previous round, made it three love sets in a row before having to come back from 3-1 down in the second.
"I didn't expect the first set to be that easy but I played unbelievable, much, much better than in my last match," the Israeli said.
The world number 17 next plays American Meilen Tu, who beat defending champion Sofia Arvidsson of Sweden 6-0, 6-2.
