PARIS, France - No.12 seed Anastasija Sevastova triumphed in a glorious, quintessential claycourt epic over No.20 seed Elise Mertens to reach the fourth round of Roland Garros for the first time, saving five match points in a riveting decider and eventually pulling through 6-7(3), 6-4, 11-9 in three hours and 18 minutes - the longest match of the tournament so far.

Sevastova saved one match point serving at 4-5 in the deciding set, two more at 5-6 and yet another pair trailing 8-9 - striking winners four times. "I didn't tell myself anything," she revealed of her mindset on those points. "I said, just move your legs and try to hit first serve and play aggressive."

One of the match point saves was a phenomenal dropshot. "Who doesn't love dropshots?" laughed Sevastova afterwards. "I love playing dropshots, but she got almost all of them - and she played them back also unbelievable. So she has hands for that." Another was a brave backhand that clipped the line - and Sevastova was philosophical about what it meant.

"In the end, we both didn't deserve to lose, I think," she concluded. "And in the end you think, was she a worse player? No. It was just a millimeter. It doesn't make you a better or worse player. In the end, it's a bit of luck. It paid off. Maybe next match it doesn't pay off, but you have to go, and I went for it."

Despite the affinity of her finesse game with natural surfaces, the Latvian had previously only reached the second week of hardcourt majors: she has reached at least the quarterfinals of the US Open for the past three years, as well as the fourth round at the Australian Open twice. This result is a breakthrough on that front, marking the first time Sevastova has progressed to the fourth round in Paris, and extends her head-to-head record against Mertens to 2-0, having previously defeated the Belgian 7-6(2), 5-7, 6-1 in the first round of Mallorca in 2017 en route to her first grass court title.

A nip-and-tuck first set pivoted on who could raise their intensity at key moments. After six relatively uneventful service holds to open the contest, things began to heat up. Mertens made the first move, winning a glorious classic clay point with a dropshot, a lob and finally a leaping stab volley - paving the way to the first break when Sevastova double faulted, and thence to a 5-3 lead as the US Open semifinalist squandered a 0-40 lead to break back with a series of backhand errors.

But Sevastova hit back despite some stellar defence from Mertens, sending down two aces and spinning a dropshot-lob web to hold - and she would not let a second straight 0-40 opportunity go begging, wheeling out the dropshot again and nailing the pass to level at 5-5.

In the ensuing tiebreak, though, an errant dropshot attempt from the 29-year-old would put Mertens ahead - and the Doha champion pressed home her advantage with strong serving and a brilliant wrongfooting forehand winner to dominate the remainder of the deciding game.

Mertens was now striking her backhand in particular beautifully, but Sevastova managed to stem the tide by upping her baseline aggression as the second set began. A brace of backhand down the lines helped the Beijing runner-up come through a tight tussle to break on her fourth opportunity for 2-1, and then as Mertens wobbled with two double faults, Sevastova pounced, firing a lightning-fast forehand return winner to seize the double break.

That lead nearly wasn't enough: Mertens is renowned for being a gritty competitor, and began to determinedly make her way to the net as she inched her way back into the set. Recovering both breaks, the 2018 Australian Open semifinalist leveled at 4-4 with a delicate counterdrop. But to Sevastova's credit, the 29-year-old held firm in the face of the comeback, deploying her renowned variety wonderfully to break again, sealing each of the next two games with a dropshot to equalize at a set apiece.

By now, both players were grooved, and the third set turned into a claycourt masterpiece. Mixing up webs of spin and slice with flatter attacks, Mertens and Sevastova used every inch of the court and every possible way to strike the ball as they sought to out-manoeuvre each other. Improvisation and invention were on full display from both, with Sevastova caressing dropshots and impossible angles at will and Mertens hustling to counter them. With both sliding around the court to shine on defence, a run of four consecutive service breaks ended with Mertens up 3-2 before each began to stamp their authority behind their deliveries again.

Leading 5-4, Mertens reached the brink of victory with a couple of moments of magic: a defensive dropshot winner flicked over the net at full stretch, and a marvelous get off a Sevastova dropshot. But Sevastova would save match point by crushing a full-blooded inside-out forehand winner - and, incredibly, both the drama and the quality would be elevated even higher as the match reached its climax.

Serving down 5-6, Sevastova saved another two match points in spectacular fashion: the second with a bold drive volley, and the third with a dropshot loaded with so much spin that it bounced sideways. The latter was a trick so good that she would repeat it immediately, this time off her forehand - sorcery that sent the crowd into raptures.

It was all a dogged Mertens could do to hold off a Sevastova who was now striking her forehand with renewed fervor - but the 23-year-old somehow fended off two break points to cling on to her next service game, and at 9-8 carved out two more opportunities to take the win. But once again, Sevastova found her best tennis with her back to the wall. A service winner saved a fourth match point, and a backhand winner down the line - indeed, on the line - saved the fifth. Just as she had six games earlier, Sevastova immediately reprised a backhand winner on the line, this time crosscourt, on the very next point.

The former World No.11's flowing aggression paid off at last in the next game. Wheeling out the dropshot repeatedly against a tiring Mertens and casually flicking follow-up winners past the Belgian, Sevastova eventually captured her opponent's serve. In front for the first time in the set, she made no mistake stepping up to the line in the next game, serving out to love in emphatic fashion and raising her tally of winners to 52 to seal a magnificent victory. Up next with a quarterfinal berth at stake is 19-year-old phenomenon Marketa Vondrousova.