Jankovic hits back to beat Keothavong
There were five breaks in the opening set but world number 96 Keothavong made the crucial one in the eighth game before serving out to take the opening stanza.
But the third seed broke her opponent in the opening game of the second set and then on two further occasions to level the match.
Keothavong saved a break point in the first game of the deciding set but the Serb then rattled off six games in a row to complete victory in 105 minutes and set up a last eight meeting with Evgeniya Rodina.
Wimbledon champion Petra Kvitova - who was today confirmed as the leader of the Czech Republic team to face Russia in the Fed Cup final next month - sealed her place in the last eight with a straight sets 6-2 6-3 victory over Patricia Mayr-Achleitner of Austria.
Kvitova was utterly relentless and unwavering as she took the opening set 6-2, and the second followed almost as comfortably as Mayr-Achleitner mustered just five games in the entire match.
The Czech will next take on Daniela Hantuchova, who was forced to work exceptionally hard to overcome Ksenia Pervak of Russia in three sets, 5-7 6-3 6-4.
Hantuchova fell a set down as Pervak seized the initiative at 5-5 in the opener, but the Slovak responded superbly to take the next two sets and seal her place in the quarter-finals.