UNTIL this week, Jo Konta listed her favourite tennis memory as playing for Britain in the Fed Cup in 2013; after ousting the Wimbledon runner-up here last night, she may just have a new one.

The 24-year-old, who until this week had won only one grand slam match in her career, produced a stunning performance to beat Garbine Muguruza of Spain 7-6, 6-7, 6-2 in three hours, 23 minutes, the longest women’s match in US Open history.

On what was perhaps the hottest day so far this week, with temperatures hitting 90 and humidity making it even worse, Konta showed impressive mental and physical endurance to get past the ninth seed.

The result matches Laura Robson’s win over the ninth seed Li Na in the third round here in 2012 but in many ways, Konta’s win was even better, matching Muguruza stroke for stroke and wearing her down in the end.

“She probably didn’t play her best today but she’s an incredible fighter and no matter what level of tennis she plays, she always brings her top-10 mentality,” said an elated and exhausted Konta, who has now won 15 matches in a row.

“I had to fight for each point and it wasn’t given to me. I’m just really happy to come back another day and have another go.”

Though Konta also beat Muguruza in Eastbourne this year, just before Wimbledon, this was a far bigger win thanks to the stage and she’ll improve her career-high ranking to around 70 but on yesterday’s effort, there could be a lot more to come.

In the past year, Konta, who spends most of her time training in Spain now, has improved beyond recognition, the technical weaknesses that blighted her game before now a thing of the past.

It took a monumental effort to get the job done as Muguruza battled for everything, saving a stack of break points to stay in touch. Having failed to serve out for the first set at 5-4, Konta took the first set on the tiebreak, 7-4 but Muguruza stayed strong to level by taking the second-set tiebreak by the same score.

At that point, Konta could easily have gone away but she remained strong, breaking at the start of the set, and despite a string of games that went to deuce, including break points, served out for the best win of her career.

“You don’t think of top players as anything different as you because then you’ve lost before you go on court,” Konta said, perhaps remembering her match here three years ago, when she failed to win from 5-2 up in the decider against Olga Govortsova in round two.

With her two wins here, added to her two ITF titles in recent weeks, Konta will go into her next match against Andrea Petkovic high on confidence but she is refusing to get carried away.

“I’m not invincible,” she said. “I’m no Serena Williams. Petkovic will be a challenge and I have to go out there and do what I can.”

Konta has given a lot of credit to her “mental coach”, Juan Coto, a Spaniard who works across many walks of life, including with accountants at PriceWaterhouse Coopers.

“There's been a lot of questions about him now,” she said. “I actually told him, and he was like, Oh, my God, in front of all the press?

“We have been doing some great work together. He's just helped me a lot with dealing with things and with enjoyment and just keeping things in perspective.

“This tennis world, this tennis community, is very much a bubble, and it's very easy to get lost in here. You know, there is a real world out there still.”

Second seed Simona Halep and former world No.1 Victoria Azarenka were among other women to make round three yesterday.