Roger Federer returned from his clay-court hiatus with a three-set victory over Mischa Zverev at the Mercedes Cup in Stuttgart.
The 36-year-old was playing his first match since losing to Thanasi Kokkinakis in the second round of the Miami Open in March, having chosen to skip the entire clay stretch for the second successive season.
Federer dropped the first set to German Zverev but recovered to win 3-6 6-4 6-2.
The Swiss needs to reach the final if he is to retake the world number one spot from Rafael Nadal, who announced his withdrawal from the tournament at Queen's Club on Wednesday.
A run to the final plus the successful defence of his title in Halle next week would see Federer ranked number one for the defence of his Wimbledon title next month.
Elsewhere on Wednesday, Indian qualifier Prajnesh Gunneswaran upset sixth seed Denis Shapovalov 7-6 (8/6) 2-6 6-3 in their first-round match and could yet play Federer should he go on to progress against Guido Pella.
Defending champion Lucas Pouille, the second seeded Frenchman, beat German wildcard Rudolf Molleker 6-3 6-4.
Denis Istomin, who had knocked out world number 22 Philipp Kohlschreiber on Tuesday, continued his good form with a 6-3 6-4 win over Florian Mayer.
Seventh seed Milos Raonic came through his first-round match against Mirza Basic 7-6 (10/8) 6-2, while eighth seeded Spaniard Feliciano Lopez progressed after Viktor Troicki was forced to retire having lost the first set 6-1.
New grass, same Fed 😎June 13, 2018
At the Libema Open in s-Hertogenbosch, third seed Gilles Muller crashed out as the Frenchman lost 6-3 7-5 to Australia's Matthew Ebden.
Sixth seed Robin Haase also failed to make an impact as he lost 6-3 7-5 to wildcard Bernard Tomic and Japan's seventh seed Yuichi Sugita went out 7-5 4-6 6-4 to Marius Copil of Romania.
Number four seed Fernando Verdasco, though, beat Daniil Medvedev 6-4 6-1.