Top seed Carlos Alcaraz survived a huge scare against Finland's Emil Ruusuvuori as his defence of the Madrid Open title began with a hard-fought comeback win.
Spain's Alcaraz, who turns 20 next week, overcame a slow start to win 2-6 6-4 6-2 against the world number 41.
There were fewer concerns for women's top seed Iga Swiatek, who won 6-3 6-2 against Austria's Julia Grabher.
However, Wimbledon champion Elena Rybakina suffered a surprise defeat to Russia's Anna Kalinskaya.
The Madrid Open is one of the most prestigious tournaments outside of the Grand Slams and an important clay-court event in the build-up to the French Open, which starts on 28 May.