TORONTO — Two straight nights, two grueling, seesaw battles.

Serena Williams took another step on her comeback injury and illness Friday, fighting off another determined opponent at the Rogers Cup.

The 29-year-old American booked her spot in the semifinals with a hard-fought 4-6, 6-3, 6-2 victory over Lucie Safarova of the Czech Republic.

The match stretched 2 hours, 4 four minutes a night after Williams had to battle back from a set deficit to beat China’s Zheng Jie, also a two-plus hour match.

Good thing the 13-time Grand Slam champ upped her fitness regimen as part of her bid to come back from being sidelined for the better part of a year after her victory at Wimbledon in 2010.

“I just decided if I can be fit, that can be a new level in my game, because I’ve always been — I think — a halfway decent player,” Williams said. “So I thought, ‘OK, what haven’t I done? I’ve never really been fit.’“