Tennis Roundup: What's wrong with Djokovic?

“I have to keep on working and trying to get better for the US Open. I just don’t feel comfortable. That’s it.”

Robredo was thrilled to win only his second match in eight encounters with a world number one player. He beat then-top ranked Lleyton Hewitt at Roland Garros 11 years ago.

“It’s amazing to be here playing and to beat the number one, I’m more than happy,” said the 32-year-old Robredo, who next faces sixth-seeded compatriot David Ferrer — a 7-5, 6-0 winner over Mikhail Youzhny.

Murray, in search of his first title since Wimbledon in 2013, has struggled to regain top form since his return from back surgery in January.

“It’s a very important match for me to win,” he said. “I had lost a few close matches over the last few months. It was important for me to come through.”

Australian Open champion Stan Wawrinka, the third seed, rallied to beat Croatian Marin Cilic 3-6, 6-0, 6-1.

He’ll play France’s Julien Benneteau, who beat Jerzy Janowicz 7-5, 6-1.

Fifth seed Milos Raonic made amends with his 30th ace on match point after earlier double-faulting three times while serving for victory.

The Canadian defeated American Steve Johnson 6-7 (7/9), 6-3, 7-6 (7/4) and next faces Italian 15th seed Fabio Fognini. Fognini beat Lu Yen-Hsun of Taiwan 3-6, 6-3, 6-3.

 – Sharapova, Halep book rematch –

 In women’s action in the combined ATP and WTA event, defending champion Serena Williams continued her march through the draw, beating Italian 13th seed Flavia Pennetta 6-2, 6-2.

Williams next plays eighth seed Jelena Jankovic, who beat Sloane Stephens 7-6 (7/4), 6-4.

Also through was world number two and second seed Simona Halep, a 6-4, 7-5 winner against Czech Lucie Safarova.

“Today was much better than my first match here,” said Halep, winner in Bucharest last month and runner-up to Maria Sharapova at Roland Garros. “I was happy to win in two sets, since she was up 5-4.

Halep earned a mouth-watering rematch with fifth-seeded Sharapova, who beat fellow Russian Anastasia Pavlyuchenkova 6-4, 7-6 (7/2).

Sharapova had to rally from a break down three times in the second set but managed to finish it off in two.

Ukrainian teenager Elina Svitolina, who took out double Wimbledon winner Petra Kvitova in the second round, claimed her second seeded victim with a defeat of Spain’s number 15 Carla Suarez Navarro 4-6, 6-4, 6-4.

She’ll next face former world number one Ana Ivanovic, a 6-2, 2-6, 6-3 winner over Svetlana Kuznetsova.

Fourth-seeded Agnieszka Radwanska of Poland cruised past Germany’s Sabine Lisicki 6-1, 6-1 and will take on Denmark’s Caroline Wozniacki, a 7-5, 6-2 winner over sixth-seeded German Angelique Kerber.