An out-of-sorts Sloane Stephens battled through to the third round of the US Open with a hard-fought victory over unfancied qualifier Anhelina Kalinina.

Defending champion Stephens racked up 42 unforced errors in a disjointed display at a searing Arthur Ashe Stadium.

But Kalinina, who herself chalked up 54 on the error count, was unable to keep up the form that saw her take out the first set, allowing world number three Stephens to eventually secure a 4-6 7-5 6-2 win.

Stephens will understand an improvement will be needed in round three when she takes on Victoria Azarenka at Flushing Meadows.

Early in the opening set, Stephens toiled between attempting to be the aggressor and showing patience in attack, while the timing of her usually potent forehand was wildly off.

Kalinina took full advantage to race into a 4-1 lead, including breaking to love in the fourth game, and it would have been worse had Stephens not fought off three break points in the sixth.

It appeared that would be the turning point as Stephens broke back with a walloping forehand but, when serving at 5-4 down, the errors crept back into her game and Kalinina thumped a winner to clinch the opener on her third set point.

However, the momentum switched to Stephens, who needed a medical timeout for a painful burst blister at 3-0 up, in the second set as Kalinina felt the heat, the American storming into a 4-1 lead.

Continued struggles on serve meant she was pegged back to 5-5 before Stephens again rallied to break when Kalinina hammered a forehand into the net.

There would be no reprieve for the world 134 this time as Stephens levelled the match, and both players had chances to break early in the decider.

Crucially, it was Stephens who made the breakthrough in game three, forcing Kalinina into a volleyed error.

With her opponent's resistance broken, the home favourite broke again in game seven before comfortably serving out for the match at the first time of asking.

STATISTICAL BREAKDOWN
Stephens [3] beat Kalinina 4-6 7-5 6-2

WINNERS/UNFORCED ERRORS
Stephens – 31/42
Kalinina – 24/54

ACES/DOUBLE FAULTS
Stephens – 2/2
Kalinina – 4/6

BREAK POINTS WON
Stephens – 7/11
Kalinina – 5/20

FIRST SERVE PERCENTAGE
Stephens – 65
Kalinina – 56

PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Stephens – 56/54
Kalinina – 55/40

TOTAL POINTS
Stephens – 110
Kalinina – 96