British No 1 Kyle Edmund suffered a collapse at the Rolex Monte-Carlo Masters after losing 12 of the last 13 games to suffer a 4-6 6-3 6-1 first-round defeat to Diego Schwartzman.
The No. 15 seed from Richmond Hill, Ont., lost 5-7, 6-3, 6-1 to unseeded German Jan-Lennard Struff in the first round of the ATP Tour Masters 1000 clay-court event on Monday.
Struff posted four aces and won 71.7 percent of his first-serve points.
Originally expected to play Dusan Lajovic before he withdrew from the 2019 Monte-Carlo Masters, the highest-ranked British men's singles player looked to have gotten off to a good start against his Argentine opponent. The pair split both of their FedEx ATP Head2Head battles a year ago, but Sousa won their only clay-court match on home soil in Estoril.
Struff next faces Grigor Dimitrov of Bulgaria, who lost in the semifinals to Rafael Nadal past year.
The 32-year-old Spaniard, whose bad knees and constant stoppages from competition have defined his career as king of clay, was extremely hesitant to even rate his chances as he gets set to face Roberto Bautista Agut in Wednesday's second round start.
Basilashvili lost 7-5, 3-6, 6-1 to Hungarian Marton Fucsovics.
After establishing his lead, Edmund proceeded to lose nine games on the trot to trail 3-0 in the final set on his way out of the tournament.
Top seed Novak Djokovic faces unseeded Philipp Kohlschreiber in the second round on Tuesday with Australia's Alex Popyrin also in action against veteran Frenchman Gilles Simon.
The 2017 semifinalist David Goffin secured his 10th victory at the tournament after overcoming Argentina's Guido Andreozzi 6-1, 6-4 in 85 minutes. Kohlschreiber stunningly upset Djokovic in straight sets during last month's BNP Paribas Open at Indian Wells.
Fabio Fognini survived an early scare in his first round match with qualifier Andrey Rublev which ended 4-6 7-5 6-4.