Signs of stress — an unruly second serve, escalating errors, the ominous cluster of clouds that darkened the sky  and a stubborn opponent who kept crunching drives into the corners —  all seemed to conspire against Serena Williams.

Pushed to the limit by 74th-ranked Caroline Garcia, Williams showed her multi-tasking skills and a fierce appetite for the fight in passing a stiff test, 6-4, 4-6, 6-4 to reach the Miami round of 16 for the 14th time.

On a muggy afternoon, the defending champion spent some time fighting herself as well as her talented  20-year-old opponent, who held leads in four of Williams' final five service games. But the world No. 1 remained committed to the cause throughout a wild match that featured five straight service breaks in the first set, nine consecutive holds in the second, careening from sputtering errors to crackling shot-making before a near 45-minute rain delay disrupted the third set.

It's unusual to see Serena drop serve twice in a set and rarer still to see her broken in succession at Crandon Park, but Garcia hammered a backhand return to break for the second time in a row and level at 3-all. Blocking back a body serve with a two-handed forehand, Williams scrambled to the open court and looped a running forehand winner to break back for 4-3. Unloading three blistering serves in a row for triple set point, Williams shrewdly hit the slider wide to set up a backhand winner, capping a 37-minute opening set as father Richard jumped out of his court-side seat in support.

Though she managed just eight games in their two previous meetings, Garcia, empowered by her recent run to the Acapulco semifinals, began to win the battle of court positioning and used her forehand to back up the six-time champion in the second set. When Williams steered a backhand beyond the baseline followed by a tight second serve into the middle of the net, Garcia gained double set point at 15-40.  Stepping in to greet a second serve with disdain, Garcia belted a backhand return down the line to level the match after 92 minutes of play. Her father and coach, Louis Paul, came out on court to provide positive reinforcement before the start of the decider.

The backhand is typically one of Serena's most reliable shots, but she wasn't always on balance in netting her two-hander several times today. Shanking a backhand that landed short in the court, Williams caught a break as Garcia got a late break on the ball and sailed a forehand sitter long to donate the break and a 1-0 lead. It was the only break of the decider.

As French Fed Cup captain Amelie Mauresmo watched from the stands, Garcia sometimes opted to indulge her craving for the rush of a quick winner rather than use her forehand to try and spread the court for a higher-percentage strike. Yet, the 2011 U.S. Open girls' runner-up dug in, continued to battle through physical exchanges, saved five more break points in the third set and sometimes stretched the defending champion with her crosscourt forehand.

Grunting louder, hitting heavier and sometimes exhorting herself between points, Williams saved break point in the second game for a 2-0 lead, overcoming a 0-30 deficit in extending the lead to 3-1. After a near 45-minute rain delay that included Gene Kelly's "Singin' in the Rain" wafting over the stadium sound system, Garcia had the unenviable task of serving down 1-3, 30-all when play resumed. She withstood three break points in that eight-minute skirmish to hold. The still-maturing Garcia is streaky but if she can learn to play the score with more consideration and construct points with more purpose, she has the tools to threaten the elite.

On a day when she struggled to sustain her range, spraying 47 unforced errors, including seven double faults, Williams' spirit was unwavering and she closed with conviction. The top seed threw down successive aces for triple match point then cracked a forehand winner crosscourt — her 34th winner of the day — screaming a rousing "Come on!" after fighting into the fourth round where she will face either 16th-seeded Samantha Stosur or American Coco Vandeweghe.