Novak Djokovic overcame physical issues to join Jannik Sinner in the Wimbledon second round, but third seed Alexander Zverev was stunned by Frenchman Arthur Rinderknech.
Seven-time champion Djokovic, who could meet Sinner in the semi-finals, began his latest pursuit of an outright-record 25th major title by overcoming Frenchman Alexandre Muller 6-1 6-7 (7-9) 6-2 6-2 on Centre Court.
The Serb appeared to struggle early in the third set - later confirming that "something was off with the stomach" - but rediscovered his form following a medical timeout to win 10 of the final 12 games.
Contesting his first Grand Slam match since losing one of the all-time great French Open finals to Carlos Alcaraz last month, world number one Sinner lost just seven games in a dominant a 6-4 6-3 6-0 win over fellow Italian Luca Nardi.
But Germany's Zverev became the fourth top-10 ranked men's player to exit the tournament across the first two days of action.
Defeats for third seed Zverev and Italy's Lorenzo Musetti came on a day when French Open champion Coco Gauff became the third top-five seed to lose in the women's draw.