Venus Williams is out of the Family Circle Cup after losing 7-6 2-6 6-4 to Canada's Eugenie Bouchard.
The 20-year-old, an Australian Open semi-finalist seeded sixth in the green claycourt tournament in Charleston, saved two set points in the first-set tiebreaker.
She then rallied from a break down in the third to advance to the quarter-finals.
Williams, who won the title in 2004, held two set points at 6-4 in the first-set tiebreaker but the highly-rated Bouchard won four points in a row to take the set.
The American battled back to comfortably take the second set and then broke for a 2-1 lead in the third.
But a string of unforced errors allowed Bouchard to recover and Williams was broken in the final game to seal victory.
"In the second set she really started going for her shots," said Bouchard.
"I just tried to keep fighting. It was a little bit ugly at times, but I just kept trying to fight through it."
Elsewhere, No 3 seed Sara Errani of Italy edged out Peng Shuai 7-6 7-6 after her Chinese opponent reached set point in each set but failed to convert.
Slovakian Jana Cepelova, conqueror of world No 1 and top seed Serena Williams earlier in the tournament, defeated No 13 seed Elena Vesnina 7-6 3-6 6-3.
Cepelova will now take on fellow Slovakian Daniela Hantuchova in the last-eight after Hantuchova made light work of defeating Teliana Pereira of Brazil 6-2 6-3.
Jelena Jankovic is now the top-ranked player after the second seed defeated Ajla Tomljanovic 7-5 6-1 however fourth-seeded Sabine Lisicki suffered a shock 6-1 6-0 rout against Andrea Petkovic.
Lucie Safarova outlasted Samantha Stosur to register a 3-6 6-4 6-4 win, while qualifier Belinda Bencic came on strong to beat Elina Svitolina 6-7 (7-4) 6-4 6-1.