You expect upsets at big tournaments, but how often do you get two of the biggest possible upsets two days in a row?

Heavily favored Serena Williams got knocked out of the U.S. Open in a semifinal match by a tall young player who had got past the early rounds at a major. Playing the day after a tough three-setter against Simona Halep, Miss Williams, the best player on the women’s Tour and dominant in the sport for almost two decades, returned powerful baseline groundstrokes — her specialty — into the net and watched winners zip by. It would have been sad, except that her aura is still so great and her spirit so strong — many players would have retired with the crick she obviously had in her left leg — that you felt pride for her mingled with the embarrassment.

But there was no denying the facts on the ground: 24-year old Karolina Pliskova held her nerves and kept pressing her advantage against the tired star. Serena Williams fought back bravely in the second set after a rout in the first, got broken, broke back, and made her way to the tiebreak. She double faulted on the last point of the match, after some fine play midway through the tiebreak when you dared hope she was getting an ultimate second wind.

This is not to take anything away from Miss Pliskova, whose power shots and superb service confirmed that her win over Angelique Kerber a few weeks ago in the Cincinnati finals, not to mention her close three-set win over Venus Williams in the quarters here, were no flukes. She is a rising power on the Tour and the rematch with Miss Kerber, who got through to the final with a win over Miss Williams’ best friend Caroline Wozniacki, could be a classic.

The women’s first semifinal match was surprising but, on reflection, not really a shocker. Serena Williams is expected to win for a reason: she usually does. But she reached the same round last year and fell short. Tennis, say what you will, is an endurance sport and sometimes you just cannot summon that last drop of will that Kipling refers to in “If.”

Roberta Vinci’s upset last year had to do as much with her own guile and cunning as with the pressure and fatigue Miss Williams carried. This year, by contrast, she faced a power hitter and big server — one of the girls who, as Miss Pliskova herself readily acknowledges, grew up idolizing the Williams sisters (Karolina’s sister Krystina did not enter this year’s U.S. Open draw) and paid them the compliment of learning their power game.

Both Williamses love the sport and love to compete. They have interests besides tennis which they take very seriously, but, in their mid-30s, they continue to insist they have years to go in the sport. And why should they not? Serena has broken more records this year, including matches won at the U.S. Open, and Venus has had an excellent season. They are strivers — a somewhat old-fashioned term that used to be heard in upwardly mobile families. You work hard and give your best, and it pays off. It will for both of them, for as long as they want, at whatever they do.

Meanwhile on the men’s side of the tournament, you could say the reverse happened, a match that was a shocker but on reflection not surprising. After his triumphs at Wimbledon and the Rio Olympics, Andy Murray was, not unreasonably, expected to win here. He was seeded second behind Novak Djokovic, whom he expected to meet in the finals. He won his first (and thus far only) U.S. Open title against the mighty Serb in 2012.

The shock was due to Murray’s huge strides (strivers, striders, who’s to say) over the past two or three years and, of course, his sensational season. He fully expects to lead Britain once again to Davis Cup victory this year, and will be heading to Glasgow to recover from Thursday’s disappointment and prepare for a meeting with an Argentine team led by Juan Martin Del Potro, one of the most admired players on the Tour for his resilience in the face of career-threatening injuries and his beautiful form.

But the loss is not particularly surprising. His opponent was Kei Nishikori, finalist here in 2014 and world No. 7. Nishikori, at 26, Nishikori is a couple years younger than Murray and, like the Scot, never gives an inch. He also concocts confounding tactics that visibly annoy his opponents, particularly nervous bundles like Murray, and when those began following an opening-set drubbing (6-1 for the Scot), you began to suspect the match would not follow the expected script.

Sure enough, Murray grew exasperated and error-prone in the course of the five-set battle. He lost the calm concentration that he started out with, and which he seems to have acquired under Ivan Lendl’s mentoring. But under the relentless shrewdness of Nishikori’s net game (they are both normally power baseliners), Murray lost his cool at crucial moments, particularly when he double faulted to give Nishikori a break opportunity late in the fifth set. Nishikori took it, then closed out the set, 7-5.

Will the change-ups and court-smarts work against Stan Wawrinka? If they do, will they help him outwit Novak Djokovic, as in the 2014 semis? Anyway, the Serb still must get past Gael Monfils. He has beaten him 13 times in a row. Can anyone beat Gael Monfils 14 times in a row?