Jankovic lost in straight sets in Dubai on Wednesday
Third seed Jelena Jankovic tumbled out of the Dubai Championships with a 6-2 7-5 defeat by Estonia's Kaia Kanepi.
"This was the worst match of my career. It was a horrible day, I am ashamed of this performance," said Jankovic.
Ana Ivanovic won 6-2 7-6 (7-5) against Camille Pin and the eighth seed will now play top seed Serena Williams, who beat 13th seed Zheng Jie 6-4 6-2.
She remains on course for a semi-final against big sister Venus, who swept to a 6-3 6-2 win over Alize Cornet.
The sixth seed will next play fourth seed Elena Dementieva, who beat Anabel Medina Garrigues 6-1 6-3.
France's Virginie Razzano beat Daniela Hantuchova 6-2 1-6 6-2 while Elena Vesnina and Vera Zvonareva both saw their opponents fail to finish their matches.
Russia's Vesnina went through when Dominika Cibulkova retired while trailing 4-6 6-1 4-0 while Zvonareva saw Marion Bartoli pull out when trailing 5-2 in the first set.
Kanepi completely dominated an off-colour Jankovic, who is a former world number one.
After trailing 6-2 4-0 Jankovic managed to pull level at 5-5 in the second set but Kanepi was not to be denied.
"I kept framing the ball, kept making unforced errors and could not put two balls together on the court," said Jankovic.
I rushed a little bit in the first set and then realised it in the second and started pulling back a little bit on my shots
Sixth seed Venus Williams
"I didn't move properly, and I didn't see the ball properly. She didn't have to do too much. Basically, I beat myself. I don't know what happened out there."
Kanepi, whose previous biggest victory came against sixth-ranked Anna Chakvedatze in last year's French Open, said: "I am really delighted. It was my tactic to mix it up and it worked.
"I did get a bit tight and started thinking too much when I was 4-0 up in the second, but such things happen when you have the opportunity to beat such a top player. Thankfully, I managed to pull it back."
Venus Williams started in a very aggressive fashion, but then adopted a more conservative approach that saw her cruise to victory against Cornet.
"It was interesting. I missed a few shots long and I was a little upset about that, but I managed to rein it in. She is obviously a really good player with a lot of potential," said Williams.
"I made more errors than yesterday. I rushed a little bit in the first set and then realised it in the second and started pulling back a little bit on my shots."