NEW YORK, NY, USA - No.3 seed Sloane Stephens was stretched to her limit before overcoming a fearless, talented qualifier in Anhelina Kalinina 4-6, 7-5, 6-2 to move into the third round, keeping her title defence alive after an enthralling two-hour, 45-minute battle.

Making her debut in a Grand Slam, today was also the first time that the Ukrainian had so much as faced a Top 40 player; hitherto, her sole Top 100 win had been over Taylor Townsend in the semifinals of the ITF $80,000 event in Charlottesville this April. Her mystique was to her advantage: "I'd literally never seen her before," admitted Stephens afterwards. "Playing someone you've never seen play before or seen what they look like, it's a little bit different, a little bit tough."

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Yet Kalinina was not remotely fazed in the biggest match of her career. The 21-year-old may have played just two WTA main draws previously, but she has experience of this particular stage: in 2014, she was the junior runner-up to Marie Bouzkova. The first three games today were dogfights, and coming through six deuces to hold her first two service games - fending off a break point in the first with a drop-half-volley - demonstrated that Kalinina had come to play.

Like Stephens, Kalinina possesses a game that is nicely balanced between offence and defence; capable of finishing points with authority off both wings, the World No.134 was also willing to come forward to good effect, winning eight of 13 net points in the first set.

Though experiments with the dropshot fared less successfully, with the Roland Garros runner-up's speed being more than equal to chasing them down, Kalinina's all-court proficiency seemed to destabilize Stephens temporarily: a sudden cascade of errors led to the loss of 14 out of 15 points, including a break to love. 

Staring down the barrel of three points to go down 1-5, the defending champion pulled a 180-degree about-turn just as abruptly. Winners suddenly flowed, particularly her favored inside-out forehand, as the American levelled the scoreline at 4-4.

At this juncture, it's all too common for the more inexperienced player to be rattled by the loss of a lead - but Kalinina never looked like crumbling. Continuing to go after her shots, it was the nine-time ITF titlist who was able to bring out the bigger hitting at the climax of the set - and the clearer thinking, too, as she adeptly finished crucial points with drive volleys rather than allowing Stephens to counterpunch. On her third set point, Kalinina ran around her backhand to smack a huge forehand return down the line - a brave decision that paid off handsomely.

One statistic did illustrate the gulf in experience between the two players, though. Kalinina would only convert five out of 20 break points across the whole match, while Stephens would manage to take seven of her 11 opportunities. The opening stages of the second set demonstrated the difference perfectly. The defending champion seized her first break point in the first game with a massive forehand, and her second in the third game by finishing a supremely entertaining rally with a dropshot out of nowhere; Kalinina, by contrast, would miss her first five chances before finally nailing a return to get one of the breaks back.

Kalinina continued to show impressive resilience despite her lost opportunities: chipping gradually at Stephens' lead as the set reached a somewhat nervy climax, with 18 unforced errors apiece for both players, resolute counterpunching and a brilliant backhand winner enabled Kalinina to break back again as the Miami champion served for the set for the first time. But she was unable to build on it: unfortunately timed errors and a double fault paved the way to a love break in the very next game, and despite a few more mishit wobbles, Stephens eventually managed to crawl over the line at the second time of asking.

"Sometimes it's not going to be the best, but you've just got to battle through," the American assessed her performance afterwards. "I just had to weather the storm and wait for my opportunities. Obviously I wish I could have been playing a little bit better. That would have helped the situation. But I just found a way today."

Stephens also admitted to nerves playing on Arthur Ashe Stadium for the first time since last year's final. "Today, I was a bit nervous," she acknowledged. "I think nervous in general - I guess getting back on Ashe for the first time. Yeah, I was a little bit, you know, uneasy."

After a 10-minute heat break - in which Stephens had gone in search of food, and found sushi to give her a boost - Kalinina returned visibly intent on shortening points, bringing out the dropshot and renewing her commitment to net approaches. But Stephens was both moving and hitting well now, and these tactics simply opened the door for the 25-year-old to show off her skills on the pass and the lob. A botched volley from Kalinina put her down a break in the third game, and at last the underdog's resistance crumbled. A tired-looking double fault and backhand error extended the deficit to 2-5 before Stephens - who had now raised her first serve percentage to 70% - served out what had been an arduous test of both nerve and skill to 15, finishing with just her second ace.

For Kalinina, though the match ended in heartbreak, the former junior standout's performance has nonetheless been an announcement of a major talent, breaking through belatedly after being sidelined for almost a year in 2016 with injuries. From a ranking low of World No.747 in January 2017, Kalinina is now closing in on the Top 100.

But it is the defending champion, whose refusal to let her US Open trophy slip from her grasp just yet overcame her fearless opponent, who lives to fight another day - setting up a tantalizing third-round clash against two-time US Open finalist Victoria Azarenka. It will be the third time the pair have faced each other this year, with Stephens winning in both Indian Wells and Miami.

"I think it will be a good match," smiled the Toronto finalist. "She's playing with a bit of confidence - and so am I."