US Open top seeds Stefanos Tsitsipas and Casper Ruud topple in New York

The 21-year-old qualifier from Switzerland, ranked 128 in the world, stunned two-time grand slam finalist Tsitsipas with a 7-5 6-7(2) 6-7(5) 7-6(6) 6-3 victory to reach the third round.

Stricker is a former French Open junior champion who recently admitted his coach had told him to cut down on chocolate and cookies but it was Tsitsipas who was left feeling sour after a four-hour slog on the Grandstand Court.

"You know, I came out today pretty well today and felt good from the first set," said Stricker.

"Over four hours it was a tough battle and I'm just super happy right now. I'm going to enjoy the rest of the day and recover tomorrow.

"It's such a great day for me and such a great win. It gives me a lot of confidence. I believe a lot in me now."

Remarkably, Stricker could have been on the plane back to Bern last week having faced a match point in the second round of qualifying.

He has now achieved his best result at a grand slam, having bettered his run to the second round at Wimbledon earlier this summer.

Tsitsipas served for the match at 5-3 in the fourth set, and the Greek said: "That hasn't happened to me before. It's the first time for me.

"Usually I'm much tougher mentally, and I have shown it by coming back. Even though sometimes I get broken serving for the match, let's say, I still find ways in the tie-break or perhaps in the fifth set. Today was not the case."

Second seed Novak Djokovic is safely through to round three after demolishing Spain's Bernabe Zapata Miralles.

The Serbian, chasing a 24th grand slam title, dropped just six games in a 6-4 6-1 6-1 victory.

There was another high-profile casualty late in the night session as fifth seed and last year's runner-up Casper Ruud went down in five sets to China's world number 67 Zhang Zhizhen, losing 6-4 5-7 6-2 0-6 6-2.

Not only does the win mark Zhang's marquee victory at a Grand Slam so far in his career, it also is a reversal of fortune from the players™ only other meeting on tour”a four-set victory by Ruud on the clay of Roland Garros earlier this year.

Zhang used his powerful serve (18 aces) and just-as-punishing forehand to pepper winners all across the court on his way to an astounding 60 winners overall. In the first set alone, the world No. 67 fired 20 winners, compared to just six unforced errors. Eight of those in the set came on aces, and through the first two sets, Zhang cranked out 11 aces to just one double fault.

That lone double fault, however, came on game point at 5-6 in the second set, the first of three consecutive unforced errors that gave Ruud the game and leveled the match at one set apiece.

Zhang recovered quickly in the third set, breaking Ruud in the first service game en route to an easy 6-2 scoreline. Ruud raised his game afterward, breaking Zhang for just the second time at the beginning of the fourth set on his way to a bagel that forced a decisive set.

Ruud once again dropped serve to begin a set, and when Zhang was given that 1-0 advantage to start the fifth, he ran away and hid from the Norwegian. More accurately, he served his way to the finish line, as Zhang dropped a total of four points on serve in the final set in a marvelous and dominant display of first-strike tennis.

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