WATCH: Denis Shapovalov booed by Basel crowd after sudden retirement

Some fans at the 2025 Swiss Indoors in Basel were unhappy when Denis Shapovalov unexpectedly retired against Joao Fonseca.

Shapovalov entered the ATP 500 tournament with some confidence after reaching the 2025 Stockholm Open semifinal. His run in Sweden was only stopped by an excellent performance from the eventual champion, Casper Ruud.

The Canadian built on that solid week by defeating Marcos Giron and Valentin Royer in Basel. Despite 19-year-old Fonseca's considerable talent, Shapovalov started as the favorite against him because of his form and overall indoor hard-court record.

Last year's Belgrade Open champion was in a good position to reach a second semifinal in as many weeks after taking the opening set 6-3. He played solidly and kept Fonseca off-balance with his hitting from the baseline.

The match turned in the second set. That was partially caused by Fonseca's improved level, who managed to bring his massive forehand into play more consistently, regularly hitting it inside-out into Shapovalov's backhand.

However, Shapovalov's level also significantly dipped. He made several surprising unforced errors, and his body language became progressively more disengaged in the second set. It felt like the 26-year-old could not wait to get off the court.

Although the 2021 Wimbledon semifinalist looked uninterested, there were no visible signs of any physical discomfort. If Shapovalov had some injury issue, he hid it well from Fonseca and the crowd.

A wild forehand from Shapovalov handed his Brazilian opponent a 4-1 and double-break lead. Fonseca loudly celebrated getting another break and marched purposefully to his chair, thinking he still needed two more games for victory.

Instead, Shapovalov appeared in front of him to shake hands, which can be seen in the video below. Fonseca was momentarily surprised before sharing some kind words with him in their exchange.

Some crowd members did not react as kindly. There were many boos and whistles when Shapovalov's handshake with Fonseca began. That continued as he packed his bags and walked off the court.

Other spectators applauded Shapovalov when he waved at them as he walked off the court. They probably believed that the former world No. 10 had a genuine injury and respected him for waving despite the booing from others.

Fonseca has a terrific opportunity to reach a maiden ATP 500 final against Jaume Munar. Many thought he made a mistake by not playing at the 2025 Shanghai Masters to focus on the indoor hard-court swing, but winning the Swiss Indoors would vindicate that move.

The hostility directed at Shapovalov might have been influenced by fellow Canadian Felix Auger-Aliassime retiring earlier in the day against Munar. He did that after losing the opening set 3-6.

Unlike Shapovalov, Auger-Aliassime looked in some discomfort before retiring. The two-time US Open semifinalist did not seem confident with his movement before deciding he could not continue.

It would be surprising if Auger-Aliassime retired for no good reason since he still has a mathematical chance of qualifying for the 2025 ATP Finals.