Japan's Kei Nishikori advanced to his first ATP semi-final in more than two years by defeating South Africa's 50th-ranked Lloyd Harris 6-3, 7-5 on Friday at the Citi Open.

Nishikori had lost six consecutive tour-level quarter-finals, including one to Harris in March at Dubai, but this time dispatched the 24-year-old, who defeated Rafael Nadal on Thursday.

"Every match I've been playing well but especially today, I played very good," Nishikori said.

"This feeling, I hope I can carry couple more weeks until US Open. This makes me great confidence. I hope I can keep going two more matches."

World number 67 Nishikori will face 107th-ranked Mackenzie McDonald, who eliminated fellow American Denis Kudla 6-3, 6-2, for a berth in Sunday's final on the Washington hardcourts.

Saturday's other semi-final will send 20-year-old American wildcard Jenson Brooksby against 24th-ranked Italian 19-year-old Jannik Sinner.

Nishikori, the 2015 Washington champion, arrived after a Tokyo Olympic quarter-final loss to top-ranked Novak Djokovic and battled jet lag when the week began.

"I was tired coming from Japan. It wasn't easy," Nishikori said. "But I got a couple days' rest and I managed to fight through the first couple rounds.

"I've been serving well this week. I make a good rhythm with my serve and that makes a huge difference."

Now the 31-year-old Japanese star, the 2014 US Open runner-up, will play in his first semi-final since April 2019 at Barcelona.

Nishikori could reach his first ATP final since taking the most recent of his 12 career titles in January 2019 at Brisbane.

Harris ousted 20-time Grand Slam champion Nadal in the Spaniard's second comeback match after a two-month layoff with a foot injury.

Japan's Kei Nishikori best South Africa's Lloyd Harris on Friday to reach the semi-finals of the ATP Citi Open
Japan's Kei Nishikori best South Africa's Lloyd Harris on Friday to reach the semi-finals of the ATP Citi Open GETTY IMAGES NORTH AMERICA / Mitchell Layton

Harris double-faulted away a break in the opening game and Nishikori broke again on his way to a 4-0 lead and claimed the set after 45 minutes.

In a back-and-forth final set, Harris saved match points with an ace and service winner but misfired on a Nishikori's third match point to fall.

"I started almost too good at the beginning," Nishikori said. "Things got tight after that. Second set was up and down. I saved well with good serve. It could go both ways but I took some chances and very happy with how I played the last game."

McDonald, seeking his first ATP final, reached his only prior semi-final in 2019 at Delray Beach.

Sinner ousted American Steve Johnson 6-4, 6-2, to become Washington's first Italian semi-finalist in 25 years. He could become the first Italian finalist in the event's 52-year history.

Sinner, a 2020 French Open quarter-finalist, seeks his third ATP title after Sofia last November and Melbourne in January. But first he must get past Brooksby, who hasn't dropped a set all week.

"He's a very tricky player," Sinner said. "He's maybe in the best moment of his young career, playing the best tennis. He's really, really in confidence."

Brooksby, ranked 130th, became the lowest-ranked Washington semi-finalist since 416th-rated John Isner in 2007 by routing Australian 11th seed John Millman 6-1, 6-2.

British three-time Grand Slam champion Andy Murray tweeted that "Brooksby is the sort of player I love to watch...lots of variety..high tennis IQ....great in defence.."

"That's great words to hear from Andy," Brooksby said. "He has been one of the best in the sport and a great guy to look up to. That means a lot."

Brooksby, runner-up last month at Newport in his first ATP final, ousted Canada's 15th-ranked Felix Auger-Aliassime on Thursday for his first victory over a top-50 foe.