Rafael Nadal put on a post-midnight winning display as he beat Mardy Fish 3-6, 6-1, 6-4, 6-2 to reach a career-best semi-final at the US Open in New York on Thursday morning.
"I'm very happy to be in the semi-finals here for the first time," said the winner, 75-8 on the season of a match which ended at 2:12 a.m. "This court has always been very difficult for me."
The Spanish No.1 and Floridian Fish took to the court well into the night after a Williams-sister marathon quarter-final.
Stubborn Serena refused to roll over against her big sister Venus, saving 10 set points in an heroic struggle before pounding out a 7-6 (8-6), 7-6 (9-7) win.
The fourth seed Serena, who lost to her sibling in the Wimbledon final, made sure that scenario did not repeat itself with an intense victory in well over two hours.
Serena, now 9-8 in the series with her sister, reached the final four in New York for the first time since 2002 when she won the title over her nearest relative.
The younger Williams next faces Russian Olympic silver medallist Dinara Safina, who stopped Italian Flavia Pennetta 6-2, 6-3.
"I think today maybe I can say finally I'm happy with myself, the way I played, because really I did what I had to do," said the sixth-seeded Russian and finalist in six of her past seven events. "I was aggressive on the court."
Serena Williams was overjoyed to have gone through against her sister, always her toughest opponent.
"It felt like a final," the eight-time Grand Slam champion said. "I can't believe I won. It's tough to play against a sister. She was serving well. I just tried to stay with her."
Fish caught fire in the early stages against Paris, Wimbledon and Olympic champion Nadal, firing four aces in the third game and sweeping the opener.
But that only served as the invitation for Nadal to shift gears, sweeping the next three sets away from the number 35 who now stands 9-9 in New York.
Nadal will next play Scot Andy Murray, who ended the 23-match winning streak of Juan Del Potro, reaching the first major semi-final of his career 7-6 (7-2), 7-6 (7-1), 4-6, 7-5.
"I'm excited to be in the semi-finals, but the tournament is still going," said the sixth seed headed for a ranking of fourth. "I said at the start of the tournament, I want to try to win it."
Nadal said beating Murray, against whom he stands 5-0, will be tough. "He's playing very well, and coming in with confidence after winning Cincinnati."
Murray laboured for nearly four hours in a high-tension shootout with Argentine teenager Del Potro, who has won titles at his past four tournaments.
Murray began fulfilling his promise as the 2004 junior winner at Flushing Meadows with a winning performance featuring nine aces and 61 unforced errors in the battle of attrition.
"I had a bit of a slip in the third set," the Scot said. "Del Potro's been on an unbelievable run."
The Argentine dropped the first two sets - around one hour each - in tiebreakers, then went on a tear as he won 12 of the 13 last points to claim the third.
He twice went up breaks in the fourth set as Murray clawed them both back before advancing in relief as a return from Del Potro sailed long on a second match point.