Borna Coric ended Roger Federer's long winning run on grass and sent Rafael Nadal back to the top of the rankings by pulling off a shock to win the Gerry Weber Open title.
Federer had won 20 consecutive matches on grass, but the top seed missed out on a 10th title in Halle, where Coric secured a stunning 7-6 (8-6) 3-6 6-2 victory on Sunday.
The 20-time grand slam champion was far from his fluid best throughout the tournament and he came unstuck against unseeded Croatian Coric, who broke two times to claim only his second ATP title and his first on grass.
Federer needed to retain his title to remain world number one, but it will be Nadal who heads into Wimbledon as the top-ranked player in the world.
Victory for Coric was his first against the Swiss great in three attempts and a huge upset given he has only won two of nine tour-level matches on grass before the tournament.
Coric did not drop a single set en route to the final, beating Alexander Zverev in his first match, and he took the first against Federer.
A tie-break was needed to separate the pair, with Federer setting up set two points with a deep forehand that Coric was unable to return.
The Croatian hit back with a powerful forehand to cancel out the mini-break, with Federer's long lob giving his opponent the first set.
Coric staved off two break points before he finally succumbed in game eight of the second. He was too delicate with a volley at the net and failed to get his shot over, with Federer serving out the set to love.
The underdog defied the odds to dethrone Federer, though, breaking for a 4-2 lead and then again to claim the title when the 36-year-old defending champion netted a forehand under pressure.