Japan’s former world No. 4 Kei Nishikori marked his return to Grand Slam tennis with a win Sunday, fending off Canadian qualifier Gabriel Diallo in five sets in the first round of the French Open. The 34-year-old Nishikori, now ranked 350th in the world, overcame his 166th-ranked opponent in a marathon 4-hour, 22-minute match at Rolland Garros 7-5, 7-6(3), 3-6, 1-6, 7-5, grabbing a vital break to seal it. “My desire allowed me to fight through at the end,” said Nishikori, whose last Grand Slam appearance was at the 2021 U.S. Open. “I’m happy with the win but mostly with the fact I’m regaining …