With major upsets highlighting the opening rounds of the 2013 Madrid Open, two-time tournament champion Rafael Nadal will face his toughest test on Friday in the quarterfinals when he squares off with compatriot David Ferrer.
Ferrer is not only the fourth-ranked men's tennis player on the planet, but a clay-court specialist who prefers the slower-playing surface.
While Nadal, the 26-year-old king of clay, has owned the head-to-head matchup with the 31-year-old Ferrer over the years—winning 19 of 23 matchups since 2004—it's not always been easy.
Similar to Nadal, Ferrer plays a relentless style of tennis and routinely showcases his impressive fitness over the course of a match—especially on clay, where the rallies tend to last much longer than anywhere else.
Half of Ferrer's 20 ATP career singles titles have come on clay, including his most recent championship in Buenos Aires back in February.
Plus, Ferrer is playing strong tennis coming into Friday's quarterfinal. He needed three sets to get past the age-defying Tommy Haas on Thursday, but was excellent in the victory, winning 72 percent of his first-serve points and 45 percent of his second-serve return points.
Ferrer will need to be sensational during his second-serve return points against Rafa to pull off the upset. So long as he continues to serve decisively, with pace and direction, he will be able to at least keep Nadal on his heels half the time.
Consider this stat from Rafa's third-round win over Mikhail Youzhny on Thursday: Nadal won 10 of 15 second-serve return points (66 percent) but just 15 of 35 first-serve return points (42 percent).
What will be the biggest key for David Ferrer vs. Rafael Nadal?
If Rafa is firing on all cylinders again on Friday, Ferrer will likely have to channel his inner Pete Sampras in order to knock out Nadal in straight sets. But it won't take anything Ferrer isn't capable of in order for him to win a set and force Rafa to break a sweat for the first time this week.
In addition to an improving serve, Ferrer possesses the foot speed to chase down balls on the clay and to cut off passing-shot attempts.
Despite his undersized 5'9" frame, Ferrer is content to outwork his opponents, opting to keep the ball in play rather than attempt a premature winner. His persistent style of play will force Nadal to bring his A-game, as he'll have to move in order to beat his Davis Cup teammate.
It would be silly to predict Ferrer to upend Nadal in Madrid on Friday, but there's no doubt that the underrated Spaniard has the necessary weapons in his arsenal to push Rafa harder than anyone else has or will in Madrid this May.
Follow Bleacher Report Featured Columnist Patrick Clarke on Twitter.