Andy Murray was the defending champion in 2014 when his 23-year-old opponent swept him out of Wimbledon.

Grigor Dimitrov stormed into his first major semifinal that day, and surely many more would be to come.

More than two years later, though, Dimitrov has yet to make it past the round of 16 at any Grand Slam since. His ranking, once in the top 10, plummeted all the way to 40th this summer, not long after he lost his opening match at five straight tournaments.

So it was quite the reversal when he won four times in the same event at Cincinnati shortly before arriving at the U.S. Open. And they all came against players ranked among the top 31 in the world — most notably Stan Wawrinka. Dimitrov wound up losing in three sets in the semifinals there to 2014 U.S. Open champ Marin Cilic.

Now the 22nd-seeded Dimitrov is in the round of 16 at a major for the first time since the 2015 Australian Open. As it happens, he lost to Murray then, and the two meet again Monday at that stage of the U.S. Open.

As it also happens, Dimitrov recently joined up with coach Dani Vallverdu, who used to work with Murray.

Once nicknamed "Baby Fed" for his game's resemblance to that of Roger Federer, Dimitrov has acknowledged that he slogged through a stretch when he wasn't liking the sport too much.

Not anymore.

"I fell in love with tennis again. It's simple as that," Dimitrov said. "It's not only because I've been winning matches or anything like that. It's because I started to enjoy the process again.

"I'm enjoying the work again. I'm enjoying waking up in the morning early to come to practice. I'm enjoying doing the ice baths again."

Dimitrov has yet to face a seeded player here and now meets an opponent who has won 36 of his last 38 matches. Murray has also won his last 26 meetings with players ranked outside the top 20. As it happens, the last such opponent to defeat him was Dimitrov. That came in Miami in March before Murray found his current form in winning his second Wimbledon title and second Olympic gold medal.

He had some shaky moments Saturday, though, splitting the first two sets against Paolo Lorenzi — who was playing in the third round at a major for the first time and coming off a five-hour match.

"I'll need to play better than today if I want to win that," Murray said after that four-set victory about his upcoming meeting with Dimitrov.

Other matches to watch on Day 8 of the U.S. Open:

WILLIAMS WATCHING: The Williams sisters play back-to-back in Arthur Ashe Stadium on Monday afternoon. Venus is up first, facing 10th-seeded Karolina Pliskova, who is in the round of 16 at a major for the first time. For Venus, it's the 45th time.

Then Serena meets 52nd-ranked Yaroslava Shvedova. The last time she lost to a player outside the top 50 at a Grand Slam was to No. 111 Virginie Razzano at the 2012 French Open, after which she started working with coach Patrick Mouratoglo. She's won nine of the 17 majors since.

COPYING CONNORS: A quarter-century after 39-year-old Jimmy Connors rode raucous crowds to an improbable U.S. Open semifinal appearance, perhaps it's only appropriate that Juan Martin del Potro could become the lowest-ranked quarterfinalist at Flushing Meadows since then.

The 142nd-ranked del Potro has been willed along by fans in New York who fondly remember his 2009 title and respect his comeback from three left wrist surgeries. He's yet to play any marathon matches, though, with three straight-set wins.

He takes on eighth-seeded Dominic Thiem, who turned 23 on Saturday and is grappling with the wear and tear of a long season.

Speaking of Connors, 37-year-old Ivo Karlovic is the oldest man to make the round of 16 at a major since that magical 1991 run. He meets sixth-seeded Kei Nishikori. Third-seeded Stan Wawrinka, who saved a match point in a five-set win Saturday, faces Illya Marchenko, who's in his first Grand Slam round of 16.

CRYING GAME: When Croatian teen Ana Konjuh played Agnieszka Radwanska in the second round at Wimbledon, she had three match points — and many more tears. Konjuh accidentally stepped on a ball and injured her right ankle in the next-to-last game of the 2-6, 6-4, 9-7 loss. Now they meet in the round of 16 under the lights at Ashe, where the fourth-seeded Radwanska will be seeking her first U.S. Open quarterfinal. The 92nd-ranked Konjuh is already on her deepest run at a major.

Fifth-seeded Simona Halep faces No. 11 Carla Suarez Navarro in the other women's match.