ROME, Italy - Seven-time Grand Slam champion Venus Williams will play fellow former World No.1 and sister Serena after a 7-5, 3-6, 7-6(4) win over Qatar Total Open winner Elise Mertens at the Internazionali BNL d'Italia.

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"It was a very long match," Venus succinctly said in her post-match press conference. "I couldn't really tell when or how it was going to end, was happy it did."

Champion at the Foro Italico back in 1999, Venus last played Serena in Rome back in 1998 for what was, at the time, their second-ever WTA meeting; the American booked a 31st clash with her sister after a whopping three hours, two minutes, and nine match points on Grandstand Court against Mertens.

Williams and Mertens played at back-to-back major tournaments in 2017 - with the former winning both encounters in straight sets. Since then, Mertens finished last season with a career-high ranking of World No.12 and won the biggest title of her career over Simona Halep in Doha

The Belgian nonetheless trailed Williams by an early break to start the match; though she turned the tables in time to serve for the opening set, the American, who was playing her first match on red clay since last year's Roland Garros, won a stirring rally when she stood two points from defeat and rode the momentum through the final three games.

"I think the first match after a while, you're just trying to hang in there, just get the motor going. I think that's really what today was about, just trying to get going."

Mertens battled back in the second set, showing poise to save all four break points faced and hit 14 winners to just five unforced errors to level the match.

As has been her hallmark over a career spanning nearly three decades, Williams took a commanding 5-1 lead in the ensuing decider, breaking serve first and securing an insurance break in a tense fifth game.

In a 12-minute marathon, Mertens gamely saved six match points at 5-2 - one with an audacious backhand overhead - and treated the Grandstand crowd to a tense ending as she broke Williams on her fourth break chance and continued narrowing the gap with a strong serve hold.

She saved an astounding seventh and eighth match points as Williams served for the match a second time, improbably levelling the match with some impressive shotmaking.

Undaunted, Williams struck a stunning backhand of her own to break back in the very next game - just clipping the line to serve for the match once more. 

Mertens again saves her best tennis for when she's behind, breaking to love and forcing a final set tie-break.

The Sudden Death proved as topsy turvy as the set itself; the two exchanged mini-breaks before the change of ends, but it was Venus who ultimately edged ahead, winning on her ninth match point.

On Wednesday, Venus will renew her rivalry with Serena for the first time since the US Open last summer, where the latter triumphed en route to her second major final of 2018. Leading their overall head-to-head at 18-12, Serena has won two of their three meetings on clay, with her only loss coming at this tournament 21 years ago.

"I know that she's going to play really well and compete really well. That's a given."

This will be their first meeting on red clay since the 2002 French Open final, which Serena won to kick off her first Serena Slam.