Forget the part-time relationship. Second-ranked Simona Halep has given coach Darren Cahill the toughest assignment in sports:

To help her surpass No. 1 Serena Williams.

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Halep announced that Cahill will become her coach in 2016, according to tennis.com. Cahill consulted for the Romanian through parts of the 2015 season, and she peaked with a semifinals appearance at the U.S. Open.

Halep had a breakout start to 2014 and appeared to be the next serious challenger to Williams' throne. She reached the finals of the French Open and the WTA Finals, as well as the semifinal at Wimbledon.

Despite her success, Halep parted ways with coach Wim Fissett, saying she wanted to work with a fellow Romanian as her coach. Halep hired Victor Ionita in 2015. She added former Maria Sharapova coach Thomas Hogstedt as a consultant but cut ties with both after Wimbledon.

Halep had a 49-17 record in 2015, winning tournaments in China, Dubai and Indian Wells, the last of which came in March. The 24-year-old suffered a lull during the middle of 2015, losing in the second round of the French Open and in the first round at Wimbledon.

Cahill coached former No. 1 men's players Lleyton Hewitt and Andre Agassi. He will continue to serve as a men's tennis analyst during Grand Slam events for ESPN.

As a player, Cahill reached No. 22 in the world and advanced to the semifinals of the U.S. Open in 1988 for his best Grand Slam result.

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