ROME (AFP) - World number one Ashleigh Barty retired from the rain-hit Italian Open on Friday (May 14) with an arm injury just two weeks before the start of Roland Garros as Rafael Nadal reached the semi-finals of a Masters event for the 75th time.

Barty was forced to pull out while leading 6-4, 2-1 in her quarter-final match against American Coco Gauff after a recurring right arm issue flared up.

Barty conceded she had not wanted to take any risks ahead of the French Open in two weeks time where she won in 2019.

"The conditions were brutal, coming on and off," she said of the rain at the Foro Italico.

"Obviously it was a little bit heavier today with the rain as well the balls were getting quite heavy.

"It's disappointing, but it's important that I listen to my body and give it time to rest and recover and make sure that in a couple weeks' time we're 100% fit for Roland Garros."

World number one Novak Djokovic's quarter-final match against Stefanos Tsitsipas was pushed back until Saturday after play was suspended because of rain, with the Greek fifth seed leading 6-4, 2-1.

Before the downpour, nine-time winner Nadal dispatched his Madrid Masters slayer Alexander Zverev 6-3, 6-4 to set up a meeting with American Reilly Opelka for a place in his 11th Rome final.

Second-seed Nadal had lost to Zverev, seeded six, in the Madrid last-eight last week, but ended his three-match losing streak against the German.

"It's a great victory for me," said Nadal after early exits in both Madrid and Monte Carlo.

The 34-year-old powered to 4-0 before Zverev held serve with the 20-time Grand Slam winner saving break point to take the first set.

The German rallied in the second set but Nadal saved eight break points to seal victory on his first match point after two hours.

"It's important because under pressure, under tough moments sometimes I lost important points the last couple of weeks.

"Today, yesterday, before yesterday too, I was able to manage it well.

"To be able to be on court against a player like Sascha, he had tough match too (yesterday), but he's more than 10 years younger than me.

"I was able to be fit, to be ready, that gives me confidence." Opelka reached his first Masters semi-final with a 7-5, 7-6 (7/2) win over Argentine qualifier Federico Delbonis.

The 47th-ranked American has not dropped his serve or a set in four games this week in which he has hit 77 aces.

Rain came to the rescue of Djokovic.

The Serbian was made to wait for 3hr 30min after play was suspended in the afternoon with Tsitsipas leading 4-3 in the first set.

The players came back on for half an hour, enough time for Tsitsipas to pull further ahead before rain came to the rescue of four-time Rome winner Djokovic.

Top seed Barty conceded "conditions were tough, without a doubt" as she played with her left thigh bandaged and a compression sleeve on her serving arm.

"I hate withdrawing," said the 25-year-old who has won a Tour-leading 27 matches in 2021.

"It's an injury that began when I was quite young, when I was 15 or 16 years old.

"It just pops up every now and again.

"I think the conditions today certainly didn't help that. But we're confident we know how to manage it, so we just kind of move on and know that the right decision was made today, as hard as it is, but we know that in a couple weeks we'll be good." Her retirement allowed 17-year-old Gauff to reach her first clay-court semi-final where she will play fifth-seed Elina Svitolina or Poland's Iga Swiatek, the reigning Roland Garros champion.

"When you have the chance to play the number one player in the world. It's not a good feeling to win win this way," said Gauff, who became the youngest WTA title winner since 2004 two years ago in Linz.

Defending champion Simona Halep, the third seed and 2018 Roland Garros winner, was also forced to pull out with a calf injury earlier this week.

Second seed Naomi Osaka exited early along with four-time Rome winner Serena Williams.

Czech Karolina Pliskova, the 2019 winner, saved three match points to set up a meeting with Croatia's Petra Martic for a place in her third consecutive Rome final.