Jelena Jankovic's bid for a fourth straight title ended on Thursday when the world number one lost 5-7, 6-3, 6-3 to unseeded Italian Flavia Pennetta in the second round of the Zurich Open.

The Serb was playing her opening match of the week after receiving a first-round bye but showed little sign of having benefited from her brief rest since winning in Moscow [Images].

"I had no gas left in the tank," Jankovic told reporters. "I am not a robot and after winning three titles in different time zones and climates I felt mentally and physically tired.

"But I will have two weeks off now to rest and train for the WTA Championships and hopefully end the year in the best possible way."

Jankovic took the first set with two breaks of serve against one dropped service game of her own but appeared to be unsettled by a painful-looking fall in the 12th game.

She received treatment for a cut on her knee after dropping serve again early in the second set only to fall further behind when Pennetta hit a return winner to go 4-1 up.

OUT OF SORTS

Jankovic broke back but continued to look out of sorts, saving one break point at 4-2 before suffering a further break at 5-3.

The third set was evenly balanced with the first seven games going to serve before Pennetta grabbed what proved to be the decisive break with a backhand winner down the line.

The Italian world number 17, who had lost all five of her previous meetings with Jankovic including last week's Moscow quarter-finals, kept her nerve to serve out the match as she finished off with an ace.

"I don't think I played that differently to when I lost to her last week," said Pennetta.

"When you're playing such a good player you have to take some risks. Sometimes you get the luck and sometimes you don't. I tried to match her style of play and I think that confused her a little."

SEEDS TUMBLE

On a bad day for the tournament's organisers Russian fourth seed Vera Zvonareva [Images] and Polish fifth seed Agnieszka Radwanska also suffered second round exits.

Radwanska was beaten 2-6, 7-6, 6-3 by Slovenian world number 21 Katarina Srebotnik in a serious blow to the Pole's hopes of qualifying for the WTA Championships.

The world number 10 is the worst-placed of the four main contenders for the three remaining spots at the year-ending tournament and is now set to lose further ground to her nearest rival Venus Williams [Images].

Zvonareva, who has yet to secure her own place at the Championships, retired from her own second round match while trailing Spain's Anabel Medina Garrigues 6-3, 3-0.

Serbian second seed Ana Ivanovic provided some relief by sailing through her second round match against French world number 16 Marion Bartoli 6-2, 6-4.

After joining Williams as one of only two seeded players to have reached the quarter-finals, Ivanovic will now take on Czech Republic's Petra Kvitova who on Thursday saw off fellow qualifier Sofia Arvidsson of Sweden 6-2, 6-1.

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