SHANGHAI, China—Roger Federer has been through a tough stretch—a sore back, an opening loss at the Masters Cup, a stomach ailment. On Wednesday, he caught a break.

Andy Roddick pulled out of the season-ending tournament hours before their match because of a sprained ankle.

Instead, Federer’s opponent was Radek Stepanek, who cut short his vacation in Thailand to compete as an alternate. The 26th-ranked Czech had to borrow rackets from Novak Djokovic because his gear shipped from home didn’t clear Chinese customs in time.

The result was a 7-6 (4), 6-4 victory ...

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