Last month, Stephens beat Keys in straight sets in the second round of the Charleston Open.

But Keys came out on top on this occasion, winning 4-6, 6-2, 7-5 in two-and-a-half hours.

It was also the longest-ever match between the pair who contested the US Open final in 2017.

France’s Kristina Mladenovic had fallen to Switzerland’s Belinda Bencic just 13 days ago in the first round of the Madrid Open.

But Mladenovic avenged that loss with a 6-4, 6-4 success to take a 4-3 lead in a head-to-head dating back to 2015.

Spanish 12th seed Garbiñe Muguruza overcame both the rain and a last-minute change of opponent as she swept into the second round.

Muguruza, a three-time Italian Open semi-finalist, was due to face Anastasia Pavlyuchenkova today, but the Russian withdrew from the event due to an abdominal injury.

Romanian Patricia Maria Tig, who lost to American Christina McHale in qualifying, replaced Pavlyuchenkova but was unable to match Muguruza who cruised to a 6-1, 6-2 victory.

"It was a weird day," said Muguruza.

"I woke up sunny, had an opponent.

"Then it turned out I had another opponent and it was raining the whole match.

"We had a little bit of everything."

Russian Veronika Kudermetova notched a hard-fought 4-6, 6-2, 6-3 victory to upset Belgian 14th seed Elise Mertens.

Kazakhstan’s Yaroslava Shvedova scored her first Women’s Tennis Association main-draw win since 2017 with a 0-6, 7-6 (7-4), 7-6 (8-6) triumph against home wildcard Martina Trevisan.

A match with world number one Ashleigh Barty is Shvedova's prize.

Roland Garros semi-finalist Nadia Podoroska also pulled off an impressive comeback to seal a second-round date with American great Serena Williams.

The Argentinian defeated lucky loser Laura Siegemund of Germany 2-6, 7-6 (7-3), 6-1 in two hours and 31 minutes.

In the men’s event, world number nine Matteo Berrettini of Italy survived a three-set battle.

Less than 48 hours after losing to Germany's Alexander Zverev in the Madrid Open final, Berrettini recorded his ninth win in his past 10 matches by beating Nikoloz Basilashvili of Georgia 4-6, 6-2, 6-4 in just over two hours.

He will now face Australia’s John Millman, who defeated Dušan Lajović of Serbia 6-3, 6-4.

Lajović's compatriot Novak Djokovic - the top seed - beat American Taylor Fritz 6-3, 7-6 (7-5) to reach the third round, another another Djokovic win was hardly the main takeaway from the match.

An angry Djokovic berated the umpire over playing conditions at one stage, unhappy with the among of rain falling on the clay at Foro Italico, and the contest was eventually suspended.

Italian Lorenzo Sonego claimed a 6-4, 5-7, 6-4 victory over France’s number 14 seed Gael Monfils.

Monfils was competing for the first time since early February when he picked up a calf injury at the Australian Open.

Sonego now challenges 10th seed Roberto Bautista Agut of Spain who posted a 6-3, 6-4 win over American qualifier Tommy Paul.

Canadian Denis Shapovalov opened his Italian Open campaign with a 6-1, 6-3 victory over Polish qualifier Kamil Majchrzak.

Reilly Opelka, who came into this Association of Tennis Professionals Masters 1000 event with just two clay-court wins to his name, has doubled his career tally so far this week.

The big-serving American struck 20 aces past Italian wildcard Lorenzo Musetti, clinching one break in each set for a 6-4, 6-4 victory over 72 minutes.

Meanwhile, Japan’s Kei Nishikori advanced to the third round without hitting a ball after Spanish 11th seed Pablo Carreño Busta pulled out of the event due to lower back pain.