Rafael Nadal is now the overwhelming favorite to win the U.S. Open after his main rival Roger Federer followed Novak Djokovic out of the tournament on Tuesday.

Federer exited the Grand Slam after a five-set thriller against Grigor Dimitrov, who came back to claim the final two sets and victory in New York.

And with Djokovic withdrawing in the fourth round, sportsbook 888 have pushed their odds on Nadal winning the U.S. Open down from +100 to -275 within 24 hours.

Why are Nadal’s odds so short?

The move to shorten Nadal’s odds into the negatives comes as Daniil Medvedev also stunned Stan Wawrinka on Tuesday night to book a semifinal showdown with Dimitrov.

Medvedev was the outsider to beat Wawrinka heading into their quarter-final clash but the Russian maintained his stellar form this summer with a victory in four sets.

Medvedev is now the only player within the top 12 seeds remaining in the tournament — bar Nadal. And this is playing into the Spaniard’s favor.

Nadal plays Argentina’s Diego Schwartzman on Wednesday, with the prize a semifinal contest against either Matteo Berrettini or Gael Monfils. Between the three of them, only Monfils has ever beaten Nadal — the last time being in 2012.

Nadal’s lack of injury concerns, the fact he has dropped just one set all tournament and the absence of Federer or Djokovic in the latter stages of the tournament all explain why his odds are so short.

Could Dimitrov cause an upset?

Yet while plenty of attention is being put on Nadal and Medvedev this week, Dimitrov has quietly gone about his business. And the Bulgarian is now in a position where he can finally meet his potential.

Dimitrov has never got past the semifinal stage of a Grand Slam, despite being hotly tipped to take over Federer’s mantle when bursting onto the ATP scene eight years ago.

His odds on winning the U.S. Open were +60000 at the start of the tournament. Betfair now have him at +800, which is still a big price for a player who has just beaten Federer.

Dimitrov must overcome Medvedev to reach the final, and the latter is the -175 favorite to win that semi with many sportsbooks. But Dimitrov has upset the odds already this week and could well do so again.