SHANGHAI (AFP) - Roger Federer laboured to an unconvincing 7-6 (7/4), 6-4 win over Czech alternate Radek Stepanek on Wednesday to keep his Masters Cup defence alive. The top seed, who is recovering from a back injury and was shocked by Gilles Simon in his first match, put in another erratic performance against the world number 27, who replaced the injured Andy Roddick. Federer started strongly as Stepanek double-faulted to be broken in the first game. But the Czech hit back before Federer took control of the tie-break, rounding it off with an unreachable forehand drop. Federer looked to have it won when he broke for 5-3, but Stepanek swiped an unstoppable backhand pass on the slide to break back. However, the recovery was brief as he double-faulted for the match in the next game. The two-time defending champion from Switzerland is gunning to become only the third man with five or more year-end titles after Ivan Lendl and Pete Sampras. Lendl completed the last hat-trick in 1987. He extends his career head-to-head to 7-2 against Stepanek, who beat the then number one at the Rome Masters in May. World number six Roddick, who rolled his right ankle on Tuesday, became the second injury withdrawal after top-ranked Rafael Nadal pulled out with tendinitis.