Andy Murray roared to victory at Wimbledon on Wednesday after being treated to a royal audience in his quarter-final clash.
The Scot booked himself a place in the semi-finals with his Centre Court victory over David Ferrer after a nail-biting tie-break.
The British number one was given a reprieve from the rain that dogged his last match, playing in dry weather apart from one brief delay, and did his now-trademark "fingers to the sky" gesture after his victory.
The Duke and Duchess of Cambridge watched from the Royal Box as six-time champion Roger Federer beat Russian Mikhail Youzhny before seeing Murray take on Spain's Ferrer.
But they were not on Centre Court to see the Scot clinch his place in Friday's semi-final against Jo-Wilfried Tsonga.
They may have had to leave early as William will tomorrow be installed as Knight of the Thistle - the highest honour in Scotland - at a special service in Edinburgh with the Queen, Duke of Edinburgh, Princess Royal and his wife.
Kate recycled an outfit she wore on the couple's tour of Canada exactly a year ago, donning a knitted sailor dress by Sarah Burton for Alexander McQueen, teaming it with navy blue heels and a Jaeger clutch bag.
The couple sat on the front row of the Royal Box - William next to Philip Brook, chairman of the All England Club, and Kate next to his wife, Gill.
They watched intently, occasionally whispering to each other, and making conversation with Mr and Mrs Brook.
Their visit completes a family set - yesterday Kate's parents, Carole and Michael Middleton, were invited into the box. Mrs Middleton was spotted on Centre Court last week, but had to sit in normal seats.
That was in contrast to Kate's siblings, Pippa and James Middleton, who were treated to the prime seats when they put in an appearance at SW19 the following day.
The Prince of Wales and Duchess of Cornwall visited last week, on Charles's first visit to the championship since 1970.
They were joined in the Royal Box by famous faces including tennis couple Andre Agassi and Steffi Graf, television sports presenter Des Lynam, veteran broadcaster Sir Michael Parkinson and England football manager Roy Hodgson.
Murray was also watched by his usual support team including girlfriend Kim Sears - spotted biting her nails during the tense match - and mother Judy Murray.
After the game, Murray said it was a tough match. He won 6-7 (7/5) 7-6 (8/6) 6-4 7-6 (7/4).
He said the delay for rain meant joining Ferrer in the locker room in what was a "tense atmosphere" as they both waited to return to the court.
William and Kate today met six-time champion Federer, who said: "They were very nice, very friendly. I was very happy to meet them.
"We had a bit of a chat. What exactly, it's not so important I don't think. But it was a nice conversation."
"I really actually do enjoy seeing legends coming out, follow today's game of tennis.
"Royals coming out, I think it's inspiring, too, in some ways, believe it or not. It's great for the event. It's great for Wimbledon. It's great for the club. It's great for the players."
Fans were joyous as the latest victory is guaranteed to send "Murray mania" into overdrive.
Monica Przygoda, 20, from London, said: "It's really exciting. I actually thought he'd lose today but it's brilliant that he won.
"It's exciting that he's through to the semi-final."
Mary and Trevor Astell, who travelled from Liverpool with son Tom, met up with daughter Roisin who is working at the championships.
Mrs Astell, 52, said: "It was fantastic. We try to come every year and we sometimes manage to get Centre Court tickets.
"It was looking a bit uncertain at times, and we knew it would be tough, but it's so great that he's through.
"We'd love to be here on Friday but we can't afford it, so we'll be watching it on the telly."