The 28-year-old Belgian, who is in his third year with Lotto Soudal, joined a 17-rider breakaway in the early going of the hot, windy 152.6km out-and-back stage, then outlasted his breakaway mates and outgunned the hard-charging peloton to take his first win since the 2016 Grand Prix Cerami.
The Argentinean race is Wallays' first since last year's Vuelta a España, a Grand Tour he raced for nearly all of the three weeks with two broken ribs and a broken wrist – although he didn't know the extent of his injuries at the time.
"In the team time trial, the first stage, I did a very good job again," Wallays said of his 2017 Vuelta. "I crashed on the second stage, but I didn't want to give up because my condition was very good.
"On the second stage we came into the feed zone and there was a bottle coming in my front wheel and I crashed," he explained. "I did a control in the hospital, but there they saw nothing, so I went to Madrid [the finish] with a lot of pain. Then when I was in Belgium I went for a control with a doctor on the team, and the conclusion was I broke two ribs and I broke also my wrist. I did the Vuelta with two broken ribs and a wrist."
Ouch. It's easy to understand why the Spanish Grand Tour was Wallays' last race of the season. Now fully recovered, he is happy to make up for lost time and start his 2018 campaign with a win.
"I've had good training in the winter, so to begin this season with this victory, for me it's a good start, and I'm looking forward to the next races," he said.
Wallays started his career with the Belgian Pro Continental team Topsport Vlaandren-Baloise, where he rode from 2010 as a trainee through 2015 before signing with Lotto Soudal. In those years he claimed wins at Paris Tours, Dwars Door Vlaanderen, Omloop van het Houtland Lichtervelde and a stage at the World Ports Classic in the Netherlands.
Wallay's role at Lotto Soudal is more of a domestique, however, and wins have been harder to come by, which makes them all the more sweet.
"What I can say about my victories is that they are always special," he said. "Most of the victories are solo, but also it's always after a very hard race. All my victories are in hard weather conditions, extremely hot or extremely rainy, extremely windy.
"They always tell me that if I was born 30 years earlier it was better to do cycling because I'd have more victories then. But now I like to work for the team and to have a good spirit in the team, and I think we showed here we have balls at the Vuelta a San Juan."
Wallays certainly showed a lot of courage during the stage. The 17-rider breakaway had to battle heat, wind, the chasing peloton and a host of escapees who did not want to share the workload.
"We had very strong guys in the breakaway," he said. "It's normally for the teams from Argentina, it's very special to be in the breakaway here. I know it because in Belgium for me it's also special, and they want to show the jersey."
The Belgian stuck with it, however, and as the peloton looked certain to bring the breakaway back, he jumped away with about 5km to go and held his advantage all the way to the line, eventually winning the stage in downtown San Juan by two seconds over Medellin-Inter's Robigzon Oyola, UnitedHealthcare's Travis McCabe, Quick-Step Floors' Iljo Keisse, and Movistar's Eduardo Sepulveda, among others. The peloton came in 12 seconds back.
The win is the Belgian team's fourth of the year following Andre Greipel's pair of sprint wins at the Tour Down Under and Tim Wellens' solo win this week at Challenge Mallorca.
Lotto Soudal are no longer an underdog in the WorldTour peloton, but rather are establishing themselves as a team to beat. In the post-stage press conference, Wallays commented on the team's new attitude.
"Last year we wanted to do it for one guy, but we changed the plan at the Vuelta, and I think there we saw a very good team," he said. "We had four victories in the Vuelta, and we are also strong enough to go early on the attack. That's one of my specialities, and I think if we need to change something it's this.
"But today it's not so easy to do it early, but last year you saw a lot of winners from very big races and I think this year is the moment for Lotto Soudal to try it."
Wallays and his teammates will have one more opportunity to try it in San Juan, with the stage 7 finale set for Sunday afternoon. The peloton will circle nine laps of the 15.7km course in downtown San Juan for 141.3km of racing. If Wallays and his team have anything to say about it, the stage will no doubt be very hard.