Rafael Nadal, right, of Spain congratulates Mackenzie McDonald of the U.S., following their second round match at the Australian Open tennis championship in Melbourne, Australia, Wednesday, Jan. 18, 2023. (AP Photo/Asanka Brendon Ratnayake)

It was a long road back to elite-level tennis for Mackenzie McDonald, and his victory over defending champion Rafael Nadal at the Australian Open showed just how much things have changed for the 27-year-old Californian who was a college star at UCLA.

To understand McDonald’s journey, go back to the 2019 French Open, where he tore a hamstring tendon three games into a first-round doubles match. Surgery for that made him unable to walk for several weeks and left McDonald with what he describes as “a massive scar.” The whole episode also served as “a big wakeup,” he says.

Returning to Roland Garros in 2020, his ranking down to 236th, McDonald got his first opportunity to face Nadal. McDonald spoke ahead of that one about having fun — then went out and lost 6-1, 6-0, 6-3.

Fast-forward to this week at Melbourne Park, where McDonald viewed his rematch against 22-time Grand Slam champion Nadal on Wednesday in a far different light than that initial meeting less than 2 1/2 years ago in Paris.

“Before, I wasn’t really up to beating a guy like that — or even believing I could,” McDonald said in an interview with The Associated Press after completing the 6-4, 6-4, 7-5 win in the second round against Nadal, who already was way behind on the scoreboard by the time he sought medical treatment late in the second set for what turned out to be an injured left hip flexor.

“I believed it more — and showed that,” said McDonald, who won NCAA titles in singles and doubles in 2016. “I was on a mission more than on a vacation.”

He twice has been to the fourth round at major tournaments, at Wimbledon in 2018 and the Australian Open in 2021.

Now ranked 65th, McDonald will try to make it at least that far yet again when he takes on No. 31 seed Yoshihito Nishioka of Japan in the third round on Court 3 in the afternoon Friday in Melbourne (sometime after 10:30 p.m. EST on Thursday).

McDonald is part of a group of U.S. men who reached the third round, many via unexpected wins, including Jenson Brooksby — who beat No. 2 seed Casper Ruud on Thursday — Tommy Paul, Ben Shelton, J.J. Wolf and Michael Mmoh.

McDonald, who is coached by 2005 U.S. Open semifinalist Robby Ginepri, used his flat groundstrokes to great effect on the slower hard court of Rod Laver Arena. The top-seeded Nadal praised the American for “playing at a great level.”

McDonald was the latest U.S. man to pick up a recent victory over Nadal, following Taylor Fritz at Indian Wells, Frances Tiafoe at the U.S. Open and Paul at the Paris Masters.

McDonald said Tiafoe told him, “Get after it!” ahead of Wednesday’s match, while Paul offered “quite a bit” of advice during a FaceTime chat the night before.

After completing a victory he considers “at the top” of his list of tennis accomplishments, McDonald spoke to folks back home.

“Everyone is super proud and pumped up for me ... but obviously, there’s another match to be played,” McDonald said, “so I have to refocus.”