Rafael Nadal's hopes of winning back-to-back US Open titles are over after his marathon tournament finally took its toll.

The Spaniard retired, citing a knee injury, after losing the first two sets of his semifinal against Juan Martin Del Potro 7-6 6-2.

The Argentine will meet Novak Djokovic after he dispatched Kei Nishikori 6-3 6-4 6-2.

Djokovic sniffing Sampras after Rafa exit

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Saturday's events at the US Open weren't insignificant when it comes to the conversation about which men are considered tennis's greatest players — and in which order.

Rafael Nadal's withdrawal robbed him of a golden opportunity to win his 18th grand slam title and move within two of Roger Federer's record of 20.

Federer's shaky exit against Australia's John Millman handed Nadal significant momentum in their head-to-head battle for GOAT status, but Nadal's injury woes continue to cost him.

Djokovic is now placed to benefit — and can match Pete Sampras' mark of 14 if he triumphs in Monday's final against Juan Martin Del Potro. The Serb has a 14-4 head-to-head mark against the Argentine.

Fears for Nishikori as carnage continues

A nasty ankle roll late in the first set has fans fearing Kei Nishikori may be forced out of his semifinal against Novak Djokovic.

After dropping the first set 6-3, Nishikori battled on in the second — saving four break points in his opening service game.

But Djokovic made the key breakthrough in the fifth game to take a 4-2 lead.

Djokovic grabbed the early break in the first set at the fifth opportunity.

Nishikori fought back from 0-40 to force deuce in his opening service game but couldn't find a way past the reigning Wimbledon champion.

Djokovic consolidated the break and took the set 6-3 in 37 minutes.

Actor Chevy Chase was enjoying the action.

Battered Nadal says No Mas

Juan Martin Del Potro is one win away from a second US Open title after Rafael Nadal was forced to retire from their semifinal.

Nadal never appeared right physically after playing several marathon matches earlier in the tournament, including a five-set epic against Dominic Thiem in the quarters.

Del Potro was in no mood to give his opponent a chance, taking a two-sets-to-love lead when the World No. 1 called a stop to proceedings.

"Of course it's not the best way to win," del Potro said. "I don't like to see him suffer like this."

Nadal had flagged his intention to pull the pin after disagreeing with a line call midway through the second set.

Nadal turned to the chair umpire and reportedly complained. "I'm going to retire anyway but you can't allow this, man," he said.

Nadal was immediately on the backfoot in the match, dropping serve in the opening game.

The Spaniard had spent more time on court than any other player in the tournament and faced question marks over whether he would be at a physical disadvantage today.

He immediately broke back and took a 4-3 lead on serve before he was broken again in the ninth game.

But despite having two set points on serve, Del Potro inexplicably blinked and Nadal broke back. "That's just a demoralising turn of events for Del Potro," tennis legend John McEnroe said in commentary for ESPN.

Both men then held serve to force a tiebreaker — and it was the Argentine looking the stronger man as he won it 7-3.

Del Potro rammed home his advantage in the second set, breaking Nadal to take a 7-6 4-2 lead as the World No. 1 began to really suffer.


Serena's husband's touching tribute

Serena Williams's husband Alexis Ohanian celebrated her return to the US Open final with a touching Twitter tribute interspersing video of her on-court exploits and the first days of their daughter Olympia a year ago.

Williams suffered life threatening complications after the birth on September 1 of last year, requiring four surgeries to deal with blood clots.

"She fought for her life, for our child, for recognition, for equal pay, for women's rights. She never gives up," Ohanian tweeted, urging fans to share the accompanying video.

"I had this made for @serenawilliams last night after her match with some home videos from 1 year ago this week," he said. "Help me make sure she sees it! #iloveyouserena"

The video contrasts shots of Williams taking the court at Flushing Meadows this week with those of her in a hospital bed, cuddling her baby daughter.

The audio is Serena's emotional on-court interview after her semi-final win over Anastasija Sevastova, when Williams called her run to a second Grand Slam final of the year "incredible".

"A year ago I was fighting for my life in the hospital after I had the baby," Williams said. "Every time I step out on this court I'm so grateful that I have the opportunity to play this sport."

Williams meets Japanese 20-year-old Naomi Osaka in the women's final on Sunday.

Bryan brother nukes Newks' record

Mike Bryan won his record-setting 18th Grand Slam doubles title and his second straight with Jack Sock, beating Lukasz Kubot and Marcelo Melo 6-3 6-1.

Bryan won his first 16 majors with twin brother Bob, who then was sidelined with a hip injury. So Mike paired with Sock and they won Wimbledon in just their second event together.

Mike Bryan broke a tie for most major doubles titles with John Newcombe, and at 40 years, four months became the oldest Grand Slam doubles champion in the professional era.

Six of his titles, also a record, have come at the US Open. Bryan and Sock, seeded third, are the first team to win consecutive Grand Slam doubles titles since Mike and Bob won four in a row from 2012-13. They are the first pair to win Wimbledon and the U.S. Open in the same year since Jonas Bjorkman and Todd Woodbridge in 2003.

Kubot and Melo, seeded seventh, were trying to win their second major together, having won Wimbledon in 2017.