Haas recovered after trailing by a set and a break of serve to win 5-7 6-2 6-3 and reach the last eight at the Italian event for the first time since 2002 when he lost to Andre Agassi in the final.
In the women's Italian Open also being played at the Foro Italico, second seed Li Na snapped her career-long jinx against Australia's Samantha Stosur, winning 6-3 6-1 having lost their previous six meetings.
The 36-year-old Haas, who reached the French Open quarterfinals last year for the first time in his career, produced some inspirational tennis to frustrate Wawrinka.
"I just tried to focus on what I was doing well and all of a sudden it clicked," 15th seed Haas told Sky Sports. "Once you are in a third set anything can happen."
Third seed Wawrinka, champion in Monte Carlo this year and fancied for a strong run at the French Open later this month, became involved in a dispute with the umpire early in the second set and his form tailed off.
Haas next plays Tomas Berdych or Grigor Dimitrov in the quarterfinals.
Also through is big-serving Canadian Milos Raonic who saw off Frenchman Jo-Wilfried Tsonga 7-6 6-4.
Top men's seed and defending champion Rafa Nadal plays Mikhail Youzhny later while women's number one Serena Williams is up against Varvara Lepchenko. - Reuters
source: gmanetwork.com
