PARIS — Andy Roddick and Juan Martin del Potro boosted their chances of qualifying for next month’s Masters Cup by winning their second-round matches in straight sets at the Paris Masters on Tuesday.

Third-seeded Novak Djokovic was leading 6-2, 4-3 against Dmitry Tursunov when the Russian retired with an injured right shoulder, seventh-seeded Roddick beat Feliciano Lopez of Spain 6-3, 6-4 and No. 9 seed del Potro beat Mario Ancic of Croatia 6-0, 6-4.

No. 16 Gael Monfils topped Juan Monaco of Argentina 6-4, 6-4 to set up a possible match against top-ranked Rafael Nadal and No. 15 Fernando Verdasco outlasted Tommy Robredo 6-2, 6-7 (6), 6-2.

Tursunov retired against Djokovic after twice needing treatment on his shoulder. Djokovic, the Australian Open champion, hasn’t won a title since his third of the year at the Rome Masters in May.

“Still trying to find my top form. Lately I’ve been struggling on the court,” Djokovic said. “But still, I’m fighting. I know that I have to fight for every point and just find my best shape.”

Djokovic has never moved beyond the third round at the Paris Masters and next plays either No. 13 Jo-Wilfried Tsonga or 2004 runner-up Radek Stepanek.

Roddick and del Potro are chasing the last three spots for the season-ending, eight-man Masters Cup in Shanghai. Roddick was sixth in the standings and del Potro seventh heading into this week’s indoor tournament at Bercy Indoor Arena.“I’m trying not to think about it,” del Potro said. “I always wake up thinking about Shanghai.”

Del Potro next faces either eighth-seeded David Nalbandian of Argentina or Nicolas Kiefer of Germany. Del Potro has split two matches against Nalbandian in the last three weeks.

Also on Tuesday:

Federer to play in Davis Cup

GENEVA — Roger Federer ended speculation over his Davis Cup plans and announced he will play for Switzerland against the United States next March. It will be the first time in five years that the 13-time Grand Slam singles winner has broken up his early season schedule for the Davis Cup. The World Group first-round series will be played March 6-8 in the U.S. at a venue to be chosen by the United States Tennis Association in the next six weeks.

Top seed Petrova advances

QUEBEC CITY — Top seed Nadia Petrova breezed past Maria Mokh 6-1, 6-2 in the Bell Challenge. Petrova will face Varvara Lepchenko in the second round. Lepchenko beat Rossana De Los Rios 6-1, 6-3. Fourth-seeded Tamarine Tanasugarn of Thailand fell to Frenchwoman Severine Bremond 6-2, 7-6. Bremond will face the winner of the Angela Haynes-Stephanie Dubois match. Fifth-seeded Aleksandra Wozniak beat Edina Gallovits 6-4, 6-3. She will face Carly Gullickson, who topped Romania’s Ioana Raluca Olaru 7-6, 6-1.