Nishikori completed a 6-2, 6-7 (8), 6-2, 6-7 (8), 7-5 victory over Benoit Paire on Monday after their Sunday meeting was suspended, and Nadal looks sure to be the more rested of the two.
Federer is back at Roland Garros for the first time since 2015 and will feature in the quarters for a 12th time. However, fellow Swiss Wawrinka will look to end his dream of making it back to the French Open final after eight years away.
Following a rush of upsets in Paris, No. 7 Sloane Stephens is now the second-highest seeded player left in the women's singles, second only to No. 3 Simona Halep.
She'll face England's Johanna Konta—who has made it past the first round for the first time in five Roland Garros appearances—while Petra Martic attempts to stop Marketa Vondrousova's fairytale run.
Tuesday's Schedule (Predicted Winner)
(3) Roger Federer vs. (24) Stanislas Wawrinka (Federer)
(2) Rafael Nadal vs. (7) Kei Nishikori (Nadal)
(7) Sloane Stephens vs. (26) Johanna Konta (Stephens)
(31) Petra Martic vs. Marketa Vondrousova (Vondrousova)
Visit RolandGarros.com to see the schedule in full.
With 12 sets won from a possible 12 thus far in Paris, both Nadal and Federer are looking relentless ahead of the quarters, but it's at this stage of the competition where the quality rises to the top.
That being said, there's simply no denying Nadal's domination when playing on clay. A flawless 6-2, 6-3, 6-3 win over Juan Ignacio Londero in the fourth round illustrated why he looks like the top pick capable of blocking Novak Djokovic's run to the crown, via the Tennis Channel:
Nadal celebrated his 33rd birthday on Monday. Tennis TV celebrated the date with a compilation of his best career moments, many of which have come on clay:
Roland Garros has a potential semi-final matchup between the Spaniard and Federer, 37. It's a Grand Slam reunion between stars who may not have many more meetings in them.
Federer's 22 career victories victories over Wawrinka, who has beaten him three times, indicates the former should be capable of topping this fixture for a seventh time in succession, per ATPTour.com.
Federer is also yet to drop a set in four matches thus far at Roland Garros, and AFP noted he and Wawrinka have a history in this stage of the French Open:
AFP Sport @AFP_Sport
Roger Federer reaches 12th Roland Garros quarter-final, beating Leonardo Mayer 6-2, 6-3, 6-3. + Record-equalling 54th Slam QF + Oldest man since 1991 in last-eight at major + Could face friend Wawrinka in repeat of 2015 QF + 'Stan beat me with his terrible shorts.' #RG19 https://t.co/ffsDZWe7mB
Wawrinka stunned his countryman 6-4, 6-3, 7-6 (4) in Paris four years ago but has since lost six times in a row to Federer.
As for Stephens, the path to a second straight French Open final is looking oddly open after a clearing out of most top seeds in the women's bracket.
That being said, the 2018 runner-up isn't looking past her upcoming date against Konta, per Eleanor Crooks of the Press Association:
And neither should she considering Konta has beaten Stephens twice in their first two matches against one another earlier this year, per SteveGTennis.com.
The most recent of those was a comeback win at the Rome Masters in May, though the American's straight-sets win over Garbine Muguruza in the fourth round suggests she's improved on clay since then.
Martic, 28, may look a firm favourite for her clash against Vondrousova based on the fact she's seeded 31st while her opponent is unranked, but the margins are closer than they appear.
Vondrousova, 19, is 38th in the WTA rankings—seven places below Martic—but she's only 158 points off her foe's total despite having played four tournaments fewer than the Croat this season.
Yet to drop a set in the competition and having already knocked out Anastasija Sevastova and Carla Suarez Navarro—seeded 12th and 28th, respectively—Vondrousova looks like good value for another upset.