Serena says she's been dealing with 'serious left knee problems'

On Thursday night, Serena Williams lost a shot at her fourth major final of 2016, as well as the chance to surpass Steffi Graf in two significant categories: Grand Slam titles, and consecutive weeks at world No. 1.

For a second straight year, Serena was upended in the US Open semis, this time falling to Czech powerhouse Karolina Pliskova. She was uncharacteristically sluggish and sloppy throughout the match, spraying 31 unforced errors in two sets, landing just 53 percent of her first serves, and double-faulting six times - including on match point.

She gave due credit to her opponent - who played with poise and purpose - but both Serena and her coach, Patrick Mouratoglou, pointed to a left knee injury that contributed to her struggles.

"I've been having some serious left knee problems," Serena said in post-match press conference. "I wasn't at 100 percent, but I also think she played well, and she deserves it."

Mouratoglou said much the same thing in an interview with ESPN after the mystifying performance.

"I can't remember a time when I've seen her move so slow," he said. ''It's impossible to compete without moving. That's why she missed so much, so many unforced errors, was because she was so slow.''

In a separate interview with Eurosport, he said: "Serena was close to giving up and retiring, but it just isn't in her DNA."

Serena came into the tournament dealing with a shoulder injury and will apparently leave with a knee injury, but for all that, she looked fit as a fiddle in her dazzling quarterfinal win over Simona Halep on Wednesday. She bristled, though, when reporters suggested that fatigue might've been a factor, given that she was playing on back-to-back days after a grueling three-setter.

"I definitely wasn't tired from yesterday's match at all," she said. "If I can't turn around after 24 hours and play another match, I shouldn't be on tour."

When she lost to Roberta Vinci in last year's semis, Serena shut it down for the remainder of the season, skipping out on the whole fall swing and the year-end championships. With no points to defend, that could now be an opportunity for her to reclaim the No. 1 ranking. That won't necessarily be enough incentive for her to play.

"Honestly," she said, when asked about her plans for the rest of 2016, "I don't know."