Serena Williams defeated Kiki Bertens 7-6 (7-5) 6-3 to reach the third round of the US Open, but it was not all plain sailing for the top seed.
The world number one kept herself on course to complete a calendar grand slam with a straight-sets victory over her Dutch opponent on Arthur Ashe.
Bertens, however, made Williams work hard in a see-saw match that twice saw the underdog waste glorious opportunities to win the opening set.
The 23-year-old - ranked 110 in the world - stunned the New York crowd when she broke serve in the third game, clinching a 2-1 lead with a superb forehand on the run.
Bertens retained her advantage through to the 10th game, giving her a chance to serve out for a 1-0 lead. However, her groundstokes deserted her just when she needed them the most, allowing the erratic Williams to get back on level terms.
The qualifier regained her composure to make it through to a tie-break, then quickly moved 4-0 up.
But, once again, Bertens could not put Williams away when she had the favourite on the ropes. The reigning champion battled back, going on to take the breaker by a 7-5 scoreline before letting out an almighty scream, such was her relief at winning the set.
The up-and-down nature of the contest continued into the second set, with Bertens losing her serve in the third game.
Although she immediately broke back, Williams surged her way ahead again by taking the fifth game.
A rare hold gave her a cushion and, despite 10 double faults and 34 unforced errors, the 21-tme grand-slam champion eventually came through the tricky test to remain on course to win all four majors in 2015.
Her next opponent will be fellow American Bethanie Mattek-Sands, who crushed compatriot Coco Vandeweghe 6-2 6-1.