Serbia's Jelena Jankovic.

Carlsbad, California – Former WTA No 1 Jelena Jankovic lost a lead of a set and along with it her concentration as the third seed crashed out in the quarter-finals on Friday at the Mercury Insurance Open to an Asian qualifier.

Numner 161 Chan Young-Jan of Taiwan earned the knockout of the Serb, who confessed that she let her chances slip away throughout the second-longest match played so far this season on the tour.

There was little drama as steady Slovak second seed Dominika Cibulkova went through in less than 90 minutes over Poland's Ursula Radwanska 6-4, 6-4.

Chan staged her upset over the course of a three-hour, 13-minute struggle, saving two match points in a second-set tiebreaker and keeping her experienced opponent on court for another two hours before advancing to the final four at La Costa.

Chan was playing in only her ninth career quarter-final and last got this far in 2009.

Chan, who has played in three semi-finals in her career, will face another huge test on Saturday against the winner from world number eight Marion Bartoli of France and American fifth seed Christina McHale.

Jankovic could not get over how she let what looked to be a win slip from her grasp.

She ended the loss as a string broke when she played on a Chan match point, which she eventually sent well long due to a lack of control on her damaged racquet.

“The court here is pretty slow, so you got to fight, you got to grind,” said the Serb who reached a grass final in Birmingham before Wimbledon to start turning around a season in which she has lost 11 times in the first round.

“The points are a little bit longer, but that doesn't have to do anything when you have so many chances like you take the lead. I complicated my life on the court. I had the match in control.

“I'm frustrated and disappointed. I completely lost focus. It was so tough mentally and physically and my forehand let me down.”

Jankovic trailed 5-2 in the final set but fought back to level and had missed chances for 6-5. “I was trying, but it didn't happen,” she said. “I had match points, I had lots of opportunities, but I didn't take them.” – Sapa-dpa